Two clocks, one slab
Concrete does not dry. It reacts. Cement grains and water form calcium silicate hydrate, and that reaction needs liquid water present inside the paste for as long as strength is still being gained. That is the first clock — hydration — and it runs at a speed set almost entirely by the temperature of the concrete itself, not the air around it.
The second clock is evaporation. Water leaves the surface of fresh concrete because the air above it is drier, warmer, or moving faster than the slab. Once the rate of loss exceeds the rate at which bleed water rises to replace it, the surface goes into tension while it still has no tensile strength worth the name. Plastic shrinkage cracking follows, usually within the first two to four hours, usually in a pattern that looks random until you notice it runs perpendicular to the wind.
Everything a crew does on a hot day or a cold one is an intervention on one of those two clocks. Fogging, sunshades and wind breaks slow evaporation. Blankets, heated enclosures and accelerating admixtures speed hydration. Curing compound seals the surface so the water already in the mix stays in the mix. Cold weather flips the problem entirely — evaporation slows on its own, but hydration slows faster, and the danger becomes freezing rather than cracking.
Sorting the work this way changes what gets argued about on site. The question is never "should we cure?" It is "which clock am I losing, and what do I have on the truck that touches it?" A superintendent who understands that will not waste a fogger on a 4°C (39°F) morning, and will not waste an insulating blanket on a 34°C (93°F) afternoon with a 25 km/h (15 mph) wind.
Reading the evaporation clock before the truck arrives
Four variables set evaporation rate: air temperature, concrete temperature, relative humidity, and wind speed. Wind is the one crews consistently underestimate. Doubling wind speed can do more damage than a 5°C (9°F) rise in air temperature, and an open slab on a bare deck with no perimeter walls is the worst case anyone routinely pours.
ACI 305R, Guide to Hot Weather Concreting, presents the relationship as a nomograph and gives a widely used threshold above which precautions against plastic shrinkage cracking are warranted. Below that threshold you are not safe, merely less exposed — mixes with low bleed rates, silica fume mixes and many fibre-reinforced mixes crack well under the classic threshold because they deliver almost no bleed water to the surface at all. For those, treat any measurable evaporation rate as a reason to fog.
Concrete temperature carries more weight in the calculation than air temperature, and it is the variable you can actually control before the pour. Cement heat of hydration then pushes it back up, so a slab placed warm on a hot afternoon may peak considerably higher in the core overnight — which matters for thermal differential, not just for surface loss.
Check the numbers the morning of, not the week before. A forecast that promised 40 percent humidity and light air is a different job when the actual is 18 percent and gusting. Reading site conditions an hour before the first truck decides whether the fog line goes out, whether the sunshade goes up, and whether the pour moves to a night placement.
Run the four site readings through this before the first truck backs in, because the decision to rig a fog line or move the pour to night has to be made while there is still time to rig it.
The temperature of the fresh concrete, not the air.
The ambient air temperature at the job site.
The relative humidity at the job site.
The wind speed across the concrete surface during placement and finishing.
Estimated evaporation rate
0.11 lb/ft²/hr
Below the critical threshold under these conditions — standard curing practices should be adequate, though conditions can change during a long pour, so re-check if temperature or wind picks up.
- Critical threshold
- 0.2 lb/ft²/hr
They open the calculator with your figures already in it
Concrete Evaporation Rate Calculator: 0.11 lb/ft²/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.11 lb/ft²/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Solar gain is not in the model. The equation reads concrete temperature, air temperature, humidity and wind only, so a slab in full sun and the same slab in shade return an identical rate from identical readings. If the sun is on the pour, take the concrete temperature off the finished surface rather than off the chute, or the answer comes back low.
- Bleed rate is not modelled. Plastic shrinkage cracking is evaporation outrunning the bleed water rising to replace it, and this calculates only the evaporation half. Low-bleed mixes - low water-cement ratio, silica fume, heavy fly ash replacement, high-range water reducers - can crack plastically at rates below the threshold this page flags.
- Nothing about the slab itself is an input. Thickness, subgrade moisture and absorption, restraint from reinforcement and edge conditions, finishing timing, and the curing method actually applied all govern whether cracks form, and none of them reach the calculation.
- This is one moment at one spot. Wind, humidity and temperature move across the hours of a placement and vary across a large slab - funnelled between buildings, faster over a hot deck, calmer behind a windbreak. Re-run it against readings taken at slab level during the pour, not against a forecast from the nearest airport.
- The threshold is a screening line, not a pass/fail verdict and not a curing specification. A rate below it is not a guarantee against cracking and a rate above it is not a prediction of it; the number tells you when to have fogging, evaporation retardant and windbreaks ready, and it does not replace the hot- or cold-weather concreting requirements in your project specification or the supplier's guidance for the mix being placed.
Conditions are favorable for a standard cure, but evaporation risk can change quickly if wind picks up or humidity drops during a long pour — recheck this calculator against updated conditions if the placement runs long.
The temperature you deliver sets both clocks
Delivered concrete temperature is the single lever that reaches both clocks at once. Warmer concrete evaporates faster and hydrates faster; colder concrete does neither. Which is why the batch plant conversation happens before the site conversation, and why arguing about fog nozzles while accepting a 34°C (93°F) delivery is arguing about the wrong thing.
Cooling on the hot side comes from chilled mix water, ice substituted for part of the mix water, sprinkled and shaded coarse aggregate stockpiles, and — on large placements — liquid nitrogen injection. Heating on the cold side comes from heated mix water and heated aggregates. Water is the easiest component to condition because it has the highest specific heat; aggregate is the hardest because there is so much of it, which is precisely why frozen lumps in a winter stockpile are a rejection item rather than a nuisance.
Specifications usually set both a maximum and a minimum as-delivered temperature, and the numbers vary by jurisdiction and by the governing project specification — ACI 305.1, Specification for Hot Weather Concreting, and ACI 306.1, Specification for Cold Weather Concreting, are the common references, but a transport authority or a local building code may impose its own limits. Confirm which document your contract cites before assuming a number.
Haul time and drum revolutions belong in the same conversation. A long haul in the heat with the drum turning adds temperature and consumes slump; retempering with water on arrival is how a specified mix quietly becomes a weaker one. Schedule the truck spacing so the crew is not tempted, and if placement genuinely cannot keep up, stop the pour rather than water the mix.
Whatever temperature you accept at the chute, a thick section keeps heating itself for hours after the finishers pack up — so look at the core-to-surface differential you are setting up while the delivery temperature is still something you can argue about.
The measured or predicted temperature at the center of the mass concrete element, in °C.
The measured or predicted temperature at the concrete surface, in °C.
Core-to-surface temperature differential
30.6 Δ°F
The core-to-surface differential and the core temperature are both under ACI 301's limits — 35 °F (19.44 °C) and 160 °F (71.11 °C) — shown with them. Those are two of the thermal control limits and they are checked against the figures entered — the pour's own temperatures are what govern, and they are measured, not predicted here.
- ACI 301 gradient limit
- 35 Δ°F
- Core temperature
- 113 °F
- ACI 301 max core temperature
- 160 °F
They open the calculator with your figures already in it
Mass Concrete Thermal Gradient Calculator: 30.6 Δ°F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 30.6 Δ°F — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This checks two temperatures you already have; it does not predict either one. Mix design, cement content, SCM replacement, placement temperature, element thickness, formwork type, insulation R-value and ambient weather play no part in the arithmetic, so it cannot tell you what gradient a pour will develop — only whether the pair of numbers you enter sits inside the limit.
- The comparison is against ACI 301's default thermal control limits, not a project-specific one. A performance-based differential derived from your concrete's tensile strength, elastic modulus and coefficient of thermal expansion can be higher or lower than the default, and that calculation, along with the thermal control plan itself, belongs to the engineer of record. This page is a screening check, not a design or an acceptance record.
- One core reading against one surface reading represents the whole element. Corners, edges, the underside against a mud slab, restraint from adjoining pours and the local gradient after formwork strip all behave differently, and none of them appear here. The answer is also only as good as where the thermocouples sit and when you read them — a surface probe at the wrong depth or a reading taken before the core has peaked changes the differential without changing anything the calculator can see.
- Nothing about the rate of change is modelled. Two pours can show the same instantaneous differential while one is cooling far faster than the other, and cooling rate is a separate control on mass concrete that this snapshot does not represent.
- The surface temperature field will not go below 0 °C (32 °F) and the core field stops at 90 °C (194 °F); a value outside those is replaced with the bound and the result is then graded on the substitute. A sub-freezing surface reading during a cold-weather placement — the case where the gradient is worst — cannot be entered as measured, and the differential it reports will be smaller than the real one.
Slowing the evaporation clock: shade, wind and water in the air
Wind breaks are the cheapest intervention and the most neglected. Snow fence, shade cloth on the perimeter scaffold, or a run of hoarding on the windward edge cuts surface air velocity and takes the sharpest term out of the evaporation equation. On a deck with no walls, temporary screens on two sides will often halve the loss rate.
Fog — not spray — is the tool for the air above the slab. A fog nozzle produces a mist that raises local humidity without landing enough water on the surface to raise the water-cement ratio at the top. Anyone who has watched a crew hose a slab "to keep it wet" has seen the soft, dusty, chalky surface that shows up six months later. Fogging goes on between screed and float, and again between float and final trowel, in the windows where the surface has no protection and no strength.
Sunshades over the working area drop the concrete surface temperature directly. On a bridge deck or a large industrial floor this is a real logistics job — frames, cloth, tie-downs that survive the same wind you are trying to block — but the alternative is a surface that sets ahead of the finishers and tears under the trowel.
Evaporation retarders sit in a category of their own. They are monomolecular films sprayed on the surface between finishing passes to hold water in until the next operation. They are not curing compounds and do not replace curing; a crew that treats them as a final coat has left the slab uncured.
Timing finishing operations also changes in the heat. Bleed water can evaporate as fast as it appears, so the surface looks ready to trowel long before the concrete underneath has stiffened. Troweling into that produces a sealed skin over a soft body, and the delamination shows up later as hollow-sounding blisters underfoot.
Sealing the slab: membrane curing and its coverage rate
Once finishing is done, the decision is water curing or membrane curing. Ponding, wet burlap and soaker hoses keep liquid water at the surface and are unambiguously the better cure for anything where surface durability governs — exposed floors, decks that will see de-icing salts, anything getting a shake-on hardener. They demand labour: burlap that dries out is worse than no burlap, because it wicks water out of the slab.
Membrane curing compound is what most jobs actually use, and its performance depends on applying enough of it evenly. ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete, and ASTM C1315, Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete, define moisture-retention performance; the manufacturer's data sheet governs coverage rate for a given product, and it is not negotiable downward to make a drum stretch. Thin, streaky application produces a slab that is well cured in stripes.
Surface texture drives consumption. A broom-finished exterior slab, a tined bridge deck or a raked surface has substantially more area than its plan dimensions suggest, and coverage rates quoted for a steel-troweled floor will not cover it. Two coats applied at right angles is standard practice on textured work, and it doubles the material take-off.
Watch what the compound does to what comes next. Many curing compounds interfere with the bond of toppings, coatings, adhesives and traffic membranes. If the slab is receiving a floor covering, either specify a dissipating-resin compound the coating manufacturer accepts, or plan on mechanical removal — which is real money and real schedule. Where a curing-and-sealing compound is used to save a step, confirm compatibility before the drums arrive, not after.
Coverage failures are invisible until the slab is already dry in patches, so convert the specified rate and the actual surface texture into drums on the truck while the order can still be changed.
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.
Speeding the hydration clock when the air turns cold
Cold weather inverts the whole problem. Evaporation stops being the headline threat — though a low-humidity winter day with dry wind still pulls water off a slab, and heated enclosures produce some of the driest air on any site. What matters instead is that hydration slows sharply as concrete temperature falls, and stops effectively near freezing.
ACI 306R, Guide to Cold Weather Concreting, sets out the operating logic: maintain the concrete above a minimum temperature for a protection period, and never let it freeze before it has reached the strength at which freezing does no permanent damage. Concrete frozen before that point suffers expansion of the pore water that disrupts the paste permanently. The slab does not recover when it thaws. That is a demolition item, not a defect to be monitored.
Practically, the cold-weather kit is heated mix water, heated aggregates, accelerating admixtures, insulating blankets, insulated forms and heated enclosures. Non-chloride accelerators are the default where reinforcement or prestressing steel is present; calcium chloride is restricted or prohibited in reinforced and prestressed work by the governing project specification and by the applicable structural concrete code in your jurisdiction, so check what your specification actually cites before ordering an accelerated mix.
Formwork is a heat asset. Steel forms strip heat out of a column; insulated forms hold hydration heat in and can carry a member through a cold night on its own reaction. Corners, edges and thin sections lose heat from multiple faces and cool fastest — a slab edge protected by a blanket laid to the perimeter and no further will freeze while the middle is comfortably warm. Blankets need to run past the edge and down the form face.
Sudden removal of protection is its own failure mode. Concrete warm inside and cold outside develops a thermal gradient, and pulling a blanket off a warm slab into a −10°C (14°F) morning cracks it as surely as any shrinkage does. Step the protection down — vent the enclosure, peel blankets progressively — so the differential across the section stays inside what the specification permits.
Direct-fired heaters carry a hazard of their own: combustion products include carbon dioxide, which carbonates a fresh concrete surface and leaves it soft, dusty and unable to take a hardener. Vent them, or use indirect-fired units where the exhaust stays outside the enclosure.
Protection has to reach past the edge and down the form face, which is always more square metres than the pour plan shows — count blankets against that wrapped area, and count them the day before, since nobody sources insulation at seven in the morning.
The total freshly-poured concrete surface area to protect.
Curing blankets needed
2 blankets
Blankets should overlap generously at seams and extend past slab edges — this estimate includes a 15% overlap allowance, but check your specific product's recommended overlap.
- Area to cover (with overlap)
- 253 sq ft
They open the calculator with your figures already in it
Concrete Curing Blanket Calculator: 2 blankets — 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
- Counts blanket area, not insulating performance. Nothing here reads ambient temperature, wind, slab thickness or cement content, so it cannot tell you whether one layer of a given blanket keeps the concrete above freezing. Hard cold, thin sections and low-heat mixes can all call for doubled-up blankets or a heated enclosure, and the count does not double.
- The 15% allowance is a flat percentage of area, so it scales with the slab and not with its perimeter. A long narrow pour — a path, a strip footing, a kerb — has far more edge per square metre than a square slab and needs more than 15% to lap the seams and hang over the sides.
- Only the area you type gets covered. Slab edges and form faces, thickened edges, curbs, and any wall or column poured with the slab are not added, and edges are where the heat goes first. The wrapped area is always larger than the pour plan area.
- Assumes one product: a 6 ft (1.8 m) x 25 ft (7.5 m), 150 sq ft roll, and the assumption does not change when the page is switched to metric. Other roll sizes, foam or hydronic mats, and blankets stripped off one pour and reused on the next all change what you actually buy.
- This is a quantity take-off, not a cold-weather protection plan. It does not set how long protection stays on, when the concrete has gained enough strength to survive freezing, or whether heated materials, enclosures, accelerators or temperature monitoring are required — that comes from your specification and cold-weather concreting guidance such as ACI 306, backed by temperature or maturity records from the actual pour.
Knowing when the clock has run out
Curing duration written as a fixed number of days is a proxy, not a measurement. Seven days at 20°C (68°F) and seven days at 4°C (39°F) are different amounts of hydration. Specifications give a nominal period — commonly seven days for many mixes, longer for slower-gaining mixes with high supplementary cementitious material content — but the underlying requirement is a strength or maturity threshold.
Maturity methods let you stop guessing. Temperature sensors embedded at the point of interest log the concrete's own thermal history, and a maturity index calibrated against cylinder breaks from the same mix gives in-place strength at any moment. ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method, covers the calibration; the calibration is mix-specific, so a change of supplier or a change of cement source invalidates it.
Field-cured cylinders remain the fallback and remain worth their trouble, so long as they are stored beside the element and protected exactly the way the element is protected. A cylinder set that spent the night in a heated site office proves nothing about the slab it came from. Acceptance cylinders — standard-cured — answer a different question entirely and should never be used to make a stripping decision.
Sensors, cylinders and blanket coverage all feed the same judgement: has enough hydration happened at the coldest, most exposed point in the pour? That point is almost never where anyone puts the probe by default. Set it at an edge, at a corner, at the top of a wall where the blanket runs short — and let the number, rather than the calendar, release the protection.
Turn the logger's temperature history into a maturity index and the argument about stripping stops being a calendar dispute; against a calibration from the same mix, it becomes a strength you can quote.
The average concrete temperature over the elapsed curing period, in °C.
The total elapsed curing time, in hours.
The temperature below which the mix gains no useful strength. ASTM C1074 recommends 0°C.
Maturity index
3,360 °C-hours
This uses a single average temperature for simplicity — a real maturity study integrates continuously logged temperature data over time, which is more accurate when temperature varies significantly during curing (e.g. cold nights, hot days). The datum belongs to your mixture: 0°C is ASTM C1074's recommendation for plain Type I cement, and Annex A1 sets out how to measure it for anything else.
- Elapsed time
- 7 days
- Datum temperature used (°C)
- 0 °C
- Temperature above datum (Δ°C)
- 20 Δ°C
They open the calculator with your figures already in it
Concrete Maturity Method Calculator (Nurse-Saul): 3,360 °C-hours — 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
- A maturity index is not a strength. Until a strength-maturity relationship has been developed for the mix actually supplied — cylinders from that mix, broken at several ages against their own logged maturity — this number cannot be read as an in-place compressive strength, and on its own it does not release formwork, tendons, sawcutting or traffic.
- Nurse-Saul sums temperature over successive intervals; this takes one average for the whole period instead. Cold nights, the hydration peak and any spell the concrete spent below the datum are all flattened into a single figure, so the wider the temperature swing over the period the further this sits from what an embedded logger would report.
- It covers one temperature, not a position in the pour. Corners, edges, thin sections and the top surface run colder than the core and reach a given maturity later, and a striking decision belongs to the coldest critical location rather than to an average of the element.
- The datum is taken exactly as entered — nothing here checks it against your cement, admixtures or supplementary cementitious materials. The Nurse-Saul function is also linear in temperature, so it does not capture the reduced ultimate strength of concrete cured hot: a warm pour can accumulate maturity quickly and still fall short at later ages, which is what the Arrhenius equivalent-age methods exist to address.
- At or below the datum the index reports zero. That is the method's bookkeeping convention, not a verdict on the concrete: it says nothing about whether a pour that got cold or froze has been damaged, and it does not cover cold-weather protection requirements.
- The index is reported in °C-hours whichever measurement system you are reading the site in, and the two temperature rows beneath it are shown in Celsius terms for the same reason: the figure is only meaningful against the strength-maturity curve it is compared to, and that curve carries the units its own cylinders were logged in. Enter your temperatures in whichever units you measured — those fields convert — but if your calibration curve is in °F-hours, multiply this figure by 1.8 before reading a strength off it.
Before the pour: what to have staged
Curing kit gets used in the first four hours or not at all. Stage it on site before the first truck, sized from actual conditions rather than the previous job's leftovers.
- Fog nozzles and hose runs — Sized to reach the far corner of the placement; fog, not spray — added surface water raises the top-layer water-cement ratio.
- Curing compound, by texture-adjusted area — Broom and tined finishes consume well above the plan-area figure; two coats at right angles on textured work.
- Wind screens on the windward perimeter — Cheapest available cut in evaporation rate; snow fence or shade cloth on the edge scaffold does most of the work.
- Insulating blankets with edge overhang — Run past the slab edge and down the form face — corners and thin sections lose heat from two or three faces.
- Embedded temperature sensors at the worst point — Coldest, most exposed location, not the convenient one; the reading is what releases the protection period.
- Non-chloride accelerator confirmed against the spec — Chloride accelerators are restricted in reinforced and prestressed work by the governing specification and code.
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.