Concrete evaporation rate and plastic shrinkage cracking field guide

The evaporation rate and the crack it predicts

Plastic shrinkage cracking happens when water leaves the surface of fresh concrete faster than bleed water comes up to replace it. The top skin dries, the drying skin wants to shrink, and the wet plastic concrete underneath holds it back. That restraint puts the weak surface in tension, and a surface with almost no tensile strength tears. The cracks open while the concrete is still plastic, during or right after finishing, before it has set hard enough to resist anything.

The surface evaporation rate is how you see it coming. It is the pounds of water evaporating off a square foot of slab in an hour, driven by four conditions on the site: air temperature, concrete temperature, relative humidity, and wind speed. The rate is not about the mix in the truck. It is about the weather over the slab and the heat in the concrete, and it can be calculated before the first yard hits the ground.

The reason this earns its own guide is timing. You cannot fix a plastic crack the way you fix a pothole, because the window to act is measured in the first hours, and most of the protection has to be staged before the truck arrives. Miss the rate on a hot dry windy day and you find out at finishing, when the cracks are already opening and the only honest options are repair or live with it. The number tells you to protect the slab while protection still works.

What is the surface evaporation rate?

The surface evaporation rate is the rate at which water leaves the exposed top of fresh concrete, expressed in pounds per square foot per hour, or kilograms per square meter per hour in metric. It is the quantity ACI 305 uses to flag plastic shrinkage cracking risk, and it depends on four things measured at the slab: air temperature, the temperature of the concrete itself, the relative humidity of the air, and the wind speed across the surface.

ACI 305 presents it as a nomograph, a chart you walk through in steps. You start at the air temperature, move up to the relative humidity line, move across to the concrete temperature, drop down to the wind velocity, and read the evaporation rate off the bottom. Online tools such as the ACPA evaporation calculator do the same math, and a published equation (Uno, in the ACI Materials Journal) reproduces the chart closely enough for field use.

Read the rate as a forecast, not a measurement. You are predicting what the weather and the concrete will do to the surface during the first few hours, so you can decide how hard to protect the slab. The nomograph and the formula give the same answer the chart was built to give. Confirm the inputs at the slab, because the wind at ground level and the concrete temperature out of the truck are what drive the number, not the airport forecast.

How do you calculate the evaporation rate?

Two ways: the ACI 305 nomograph, or the published formula that fits it. The nomograph is the standard reference and the one most specs point to. When you want a number without the chart, the Uno equation gives a close match and is easy to put in a spreadsheet or a phone. Both take the same four inputs, and both spit out a rate you compare against the threshold.

The formula runs in metric and gives kilograms per square meter per hour. Concrete surface temperature and air temperature go in as degrees Celsius, relative humidity as a decimal fraction, and wind speed as kilometers per hour. The result converts cleanly: 1.0 kg per square meter per hour is about 0.2 lb per square foot per hour, which is the same as the ACI action threshold. Multiply a metric rate by 0.205 to get the customary units, or just remember that 1.0 metric equals the 0.2 customary flag.

The piece people get wrong is the concrete temperature. It is not the air temperature, and on a hot day fresh concrete often runs several degrees above ambient out of the drum, which pushes the rate up faster than the air alone suggests. Measure the concrete temperature per ASTM C1064, the same reading you take with the slump set, and feed that in. Guess it and the rate is wrong in the direction that gets you hurt.

E

Evaporation rate, kg per square meter per hour from this equation; convert to lb per square foot per hour for the ACI threshold

Tc

Concrete surface temperature in degrees Celsius, measured per ASTM C1064, not the air temperature

Ta

Air temperature in degrees Celsius, measured in the shade at the slab

r

Relative humidity as a decimal fraction, so 40 percent is 0.40

V

Wind speed in kilometers per hour, measured at about 20 in above the surface

What evaporation rate causes plastic shrinkage cracking?

ACI 305 recommends taking precautions against plastic shrinkage cracking when the evaporation rate is expected to approach 0.2 lb per square foot per hour, which is 1.0 kg per square meter per hour. That is the number to carry in your head. It is the action threshold, the point where you stage protection rather than hope.

The guidance reads as a band, not a cliff. Below about 0.1 lb per square foot per hour, plastic shrinkage cracks are generally not expected. Between 0.1 and 0.2, they may occur, and that middle ground is where sensitive mixes get caught. Above 0.2, cracking is expected unless you protect the surface. So 0.2 is the line where action is clearly called for, but it is not a guarantee of safety below it.

Drop the action point for mixes that bleed little, which is most modern concrete. Low water to cement ratio mixes, mixes heavy in fly ash, slag, or silica fume, and any mix designed to bleed slowly can crack at rates well under 0.2, because there is little or no bleed water to refill the surface. On those mixes, treat 0.1 as the number that should put you on alert, and verify the action level against the project specification and the mix supplier's guidance. The threshold is a screening tool, not a pass.

Why concrete temperature and wind dominate the number

Four inputs drive the rate, but two of them carry most of the weight: the concrete temperature and the wind. People expect air temperature and humidity to be the whole story, then get surprised by a moderate day that still cracks. The day that catches crews is hot, dry, and windy, and it is the wind and the warm concrete doing the damage.

Wind is the predominant factor in fast surface water loss. Still air over a slab builds a thin humid layer right at the surface that slows evaporation on its own. A 15 mph wind strips that layer away and replaces it with dry air, over and over, so the surface dries far faster than the same conditions with no wind. A windbreak that does nothing but stop the air movement can pull a rate from cracking-expected back under the line by itself.

Concrete temperature is the other lever, and it climbs fast because the formula weights it heavily and because fresh concrete runs hotter than the air on a hot pour. The cement is hydrating and giving off heat, and the truck has been baking. A concrete temperature 5 to 10 degrees above ambient is common, and that gap alone can move the rate from a watch to a problem. This is the case for cooling the mix and for measuring the concrete temperature instead of assuming it equals the air.

What does plastic shrinkage cracking look like?

Plastic shrinkage cracks are short, shallow, and scattered across the open top face of a slab, and they show up during or just after finishing. They run a few inches to a few feet long, often in a random map but sometimes as roughly parallel cracks spaced inches to feet apart. When they line up parallel, they tend to sit perpendicular to the wind that caused them, which is a useful tell on a windy pour.

They are surface cracks. Widths at the top can reach about 1/8 in, but they taper fast with depth and usually run only a fraction of an inch deep, though a bad case can go deeper. They appear early, commonly in the first 1 to 6 hours while the concrete is still plastic, which is the feature that names them and separates them from cracks that come later.

Tell them apart from the other two early cracks by timing and shape. Drying shrinkage cracks form days to weeks out as the hardened concrete loses moisture and pulls against its restraint, and they are the ones control joints are cut to manage. Plastic settlement cracks form over the top of rebar or at thickness changes, as the fresh concrete sinks around an obstruction and the surface telegraphs it. Plastic shrinkage cracks are the early, shallow, surface-only map cracks driven by evaporation, and the cure for them is moisture control on the fresh surface, not joints and not rebar cover.

Bleed water and the two ways the surface fails

Bleed water is the water that rises to the surface after placement as the solids settle. While it sits there, it protects the surface from drying, because the air is taking the bleed water and not the water bound in the paste. The trap is that bleed water creates two opposite failure modes depending on the mix, and the same crew can get bitten by either one.

Failing mode one: finishing while bleed water is still on the surface. If a finisher floats or trowels water back into the top, the surface ends up with a high water to cement ratio right where wear and weather hit it. You get a weak skin that dusts, scales, and crazes later. The rule finishers learn the hard way is to wait until the bleed water has left the surface or been removed before hard finishing, and never to work standing water back in.

Failing mode two is the modern problem and it is the one this guide is about. A low-bleed mix barely brings water to the surface at all, so there is nothing to protect the skin and nothing to refill what evaporates. On a hot windy day that surface dries and cracks fast, while the finisher is still waiting for bleed water that is never coming. So the old instinct of waiting for the surface to clear can hold a crew on a slab that is already drying and tearing underneath their wait. Two failure modes, opposite causes, and you have to know which mix you are standing on.

Why modern mixes crack more, not less

The mixes that are stronger and more durable also bleed less, and low bleed is exactly what feeds plastic shrinkage cracking. Three things drive it: low water to cement ratios, supplementary cementitious materials, and the fine, dense paste those produce. The water that used to rise and protect the surface is not there to rise.

Supplementary cementitious materials are part of nearly every spec now, for good reasons. Fly ash, ground granulated blast furnace slag, and silica fume cut cement, improve durability, and lower heat in mass placements. They also make the paste finer and the bleed slower or smaller. Silica fume mixes are notorious for almost no bleed, which is why they crack plastically so readily that fogging and retarders are often written straight into the spec.

Low water to cement ratio does the same thing from a different angle. Less mixing water means less free water, so less bleed, and the high-performance mix that meets the strength and durability targets is the same mix that has little surface water to spare. The lesson is to flip the old assumption. A rich, low-slump, SCM-heavy mix is not safer on a hot windy day. It is more exposed, and it deserves the protection earlier and harder than a plain mix would.

Field example: a hot dry windy afternoon

Run a real set of conditions through the rate. The air is 90 degrees F, the concrete comes off the truck at 95 degrees F, the relative humidity is 30 percent, and the wind is 15 mph across an open slab. Convert and run the formula and the rate lands around 0.43 lb per square foot per hour, more than double the 0.2 action threshold. This pour cracks unless you protect it.

Now change one input at a time and watch the wind dominate. Put up a windbreak that knocks the wind down to about 5 mph and the rate falls to roughly 0.19, back under the line, with nothing else changed. Stopping the air did most of the work. Instead leave the wind at 15 and fog the air to bring the local humidity up near 70 percent, and the rate drops to about 0.23, better but still on the wrong side. On the worst day no single measure is a cure. You stack them.

The point is not the exact decimals, which shift with how you measure each input. The point is the size of the moves. A windbreak alone cut the rate by more than half here, and that is the cheapest protection on the truck. Cooling the concrete a few degrees and fogging on top of the windbreak would put this slab in a comfortable place. Skip all of it and the finishers meet the cracks at the worst possible moment.

The protective measures, ranked by what they cost you

Once the rate says protect, you have a set of measures that all do the same job from different angles: cut evaporation off the surface, or cut the conditions that drive it. The art is staging them before the truck, because every one of them is useless once the cracks have opened.

Windbreaks and sunshades attack the conditions directly. A windbreak is the highest-value, lowest-cost move on most sites, because wind dominates the rate and a barrier of fence fabric or plywood on the upwind side cuts it cheaply. Sunshades knock down the radiant heat that drives the concrete and surface temperatures up. Cooling the mix is the supply-side version: chilled water, ice replacing part of the mix water, or shaded aggregate, ordered from the plant so the concrete arrives cooler. Scheduling is the free one. Move the pour to early morning, evening, or night and you sidestep the peak of heat, sun, and afternoon wind entirely.

Then there are the measures that work on the surface itself. Fogging raises the humidity in the air just above the slab. Evaporation retarders lay a thin film on the surface that slows water loss between finishing passes. Some specs add an evaporation-reducing admixture batched into the mix. These three are covered in the next sections, because they are the ones most often misused. The order to reach for them is roughly: schedule around the heat, break the wind, shade and cool, then fog and retard the surface. Stack them on the worst days.

Can you fog concrete?

Yes, and on a high-evaporation pour fogging is one of the most effective things you can do, as long as you fog the air and not the slab. Fogging means putting a fine mist into the air just above the surface with a fogging nozzle, raising the local relative humidity so the air over the slab pulls less water out of the concrete. Done right, the mist hangs above the surface and almost none of it lands on the concrete.

What you do not do is hose or spray water onto the surface to keep it wet. Water added to a plastic surface and then worked in raises the water to cement ratio of the skin, and you trade a crack for a weak, dusting, scaling surface. The distinction is the whole point. Fogging adds humidity to the air. It does not add water to the concrete. A finisher who sprays the slab down because it is drying has created a finishing defect to avoid a cracking defect.

Use a real fogging nozzle that atomizes to a true mist, not a garden spray that throws droplets. Keep it up between finishing operations and through the vulnerable early period when bleed is light and the surface is exposed. On a silica fume or other near-zero-bleed mix, fogging is often the difference between a clean slab and a cracked one, and it is commonly specified outright for those mixes. It buys time. It does not replace curing once the surface is finished.

Evaporation retarder vs curing compound

An evaporation retarder and a curing compound are different products for different stages, and using one as the other is a common and costly mix-up. An evaporation retarder is a sprayed-on monomolecular film, a very thin layer that sits on the fresh surface and slows water loss while you are still finishing. A curing compound is a membrane sprayed on after finishing to hold moisture in during the days of curing that follow.

The retarder protects the plastic concrete during finishing. You apply it after strikeoff and again between finishing passes if the surface is drying, on the hot windy days when fogging and windbreaks are not keeping up. It buys the surface time against evaporation until you can finish and then cure. It is not a finishing aid to be worked into the surface, and it is not curing.

Here is the line that has to be clear: an evaporation retarder is not a curing compound and is not a substitute for curing. The retarder is gone or absorbed by the time curing matters, and it was never built to hold moisture for days. Spray a retarder, finish the slab, and then cure it properly with a curing compound, water, or coverings. Treating the retarder as the cure is one of the more expensive shortcuts in flatwork, because the slab looks protected and is not.

Hot weather concreting (ACI 305), and a cold-weather note

ACI 305 is the guide for hot weather concreting, and the evaporation rate sits inside it as one piece of a larger plan. The rate is one input. Hot weather work also means controlling the concrete temperature, the placing and finishing speed, and the protection, all decided before the pour. The hot day shortens every working window, and the plan exists because reacting in real time is too slow.

There is a maximum concrete temperature, and the number depends on which document and which edition you read. A maximum placement temperature of 90 degrees F shows up in many older specs and is still common in project documents. The ACI 305.1 specification raised the general discharge limit to 95 degrees F in its 2014 edition, with mass concrete held tighter, often around 85 degrees F. The honest field rule: hold the concrete temperature to what the project specification says, confirm the edition that governs, and do not assume a number from memory.

Truck timing is the part the rate makes urgent. The longer a load sits in the heat, the warmer and stiffer it gets, and the higher the surface evaporation runs once it is placed. Sequence the trucks so concrete is not waiting in the sun, keep the discharge moving, and cut the time from batch to finish. The slump test guide covers the batch-to-discharge clock and the on-site water rules, which run on the same hot-weather pressure as the evaporation rate does.

One cold-weather caveat, because plastic shrinkage cracking is not strictly summer. A cold, dry, windy day with sun and concrete warmer than the air can still drive the rate up, since the formula cares about the gap between the concrete and the air, not the season. Cold weather concreting is otherwise a different fight, protecting the mix from freezing and never pouring on frozen subgrade, and those provisions live in ACI 306. The crossover lesson: run the rate on any low-humidity, windy pour, and do not assume a cold day is safe from surface drying.

What is the difference between curing and evaporation control?

Evaporation control protects the concrete while it is plastic, in the first hours. Curing protects it after it has set, over the days that follow. They overlap at the handoff but they are not the same job, and a slab needs both. Confuse them and you protect the slab at the wrong time with the wrong method.

Plastic-stage protection is fogging, retarders, windbreaks, and shade, all aimed at one thing: keeping the surface from drying and tearing before the concrete has any strength. This is the window where plastic shrinkage cracks form, and once the concrete sets, that specific risk is over. The measures that fight it stop mattering the moment the surface can resist tension.

Curing starts at finishing and runs for days, and its job is to keep enough moisture in the set concrete for the cement to keep hydrating and gaining strength. That is curing compound, wet burlap, ponding, plastic sheeting, or curing blankets, covered by ACI 308. A slab that was fogged and retarded perfectly through finishing and then left to dry will still come up weak and crazed, because nobody cured it. Protect the plastic concrete, then cure the set concrete. Both, in order, every time.

Measuring and monitoring on the pour

The rate is only as good as the four inputs, so measure them at the slab, not from a forecast. A jobsite weather meter reads air temperature, relative humidity, and wind speed in one device. Hold it at the surface, in the shade, and read the wind low where the slab actually sees it, not over your head. The airport wind ten miles away is not the wind on your open deck.

Concrete temperature gets measured per ASTM C1064, the same reading taken with the slump and air tests at acceptance. Push the sensor into the fresh concrete to the depth the method calls for and give it time to settle. This is the input people skip and the one that moves the rate hard, because fresh concrete runs warmer than the air on a hot pour. The slump test guide covers the C1064 reading inside the fresh-test set.

Compute the rate twice. Once in planning, against the day's forecast, so you know what to stage and whether to move the pour. Then again at the slab when the concrete arrives, with the real numbers, because the forecast is never exactly the deck. A tool that runs the four inputs through the nomograph math and flags when you cross the threshold during placement turns this from a one-time check into a live gate. Recompute as the wind picks up or the sun comes around, since the conditions that drive the rate change through the pour.

The pour-day go or no-go

The decision is made before the truck, not at the chute. Run the rate against the forecast the day before and the morning of. If it is comfortably under the threshold for the mix, pour and keep an eye on conditions. If it is near or over, you either move the pour or stage the protection, and staging means the gear is on site and ready, not on a list.

Have the protection physically present before placement starts: fogging equipment hooked up and tested, evaporation retarder mixed and in the sprayer, windbreak material up on the upwind side, sunshades positioned, and curing materials ready for the handoff. The whole reason this is a planning problem is that you cannot improvise a windbreak while the slab is cracking. The window is too short.

Be willing to call it. A pour that runs well over the threshold for a low-bleed mix, with no way to break the wind or cool the concrete, is a pour that should move to night or early morning rather than go in and crack. That is a real superintendent call with real schedule cost, and it is cheaper than a cracked deck and a repair argument. Decide it on the rate and the staged protection, and write down what drove the call.

Repairing plastic cracks: the window and the lock-in

There is a short window to close a plastic crack and a long time after it where you cannot. While the concrete is still plastic and responsive, a crack that opens during finishing can often be closed by re-vibrating, re-striking, or re-troweling across it, working the fresh paste back together so the crack heals before set. That only works while the concrete still moves. Catch it early, work it closed, and protect the surface so it does not reopen.

Once the concrete has set, that window is gone and the crack is locked in. Now it is a hardened crack to be evaluated and repaired on its merits, not finished out. Shallow plastic cracks are often cosmetic and shallow enough to leave or treat with a surface repair, but they can be a path for water and a durability concern depending on the slab and the exposure. Routing and sealing, or a low-viscosity filler, are typical repairs once set, per the project's repair requirements.

The honest framing is that repair is the failure case. Re-troweling a few cracks closed in the window is recovery from a near miss. A slab that set with a map of plastic cracks across it is telling you the protection was not staged or not enough, and the repair is more expensive and less convincing than the windbreak and the fog would have been. The cheap fix is on the front end.

What to document

When a slab crazes or cracks plastic and the finger-pointing starts, the only thing that shows you read the conditions, ran the numbers, and staged protection ahead of the pour is what you logged at the time. The record is what shows the conditions were checked, the rate was computed, and the protection was staged, before anyone went looking for blame. Write it at the pour, against the clock, not from memory after.

Capture the time of each reading, the air temperature, the relative humidity, the wind speed, the concrete temperature, and the computed evaporation rate, plus what you did about it. Log the protective measures actually in use: windbreak up, fogging on, retarder applied, mix cooled, pour rescheduled. If conditions changed through the pour, log the recompute. Tie the curing method and start time to the same record, because the inspector reading it later wants to see the plastic protection and the curing both happened, in order.

Common mistakes

• Pouring a hot dry windy day with no evaporation-rate check, then meeting the cracks at finishing.
• Feeding air temperature into the rate instead of the measured concrete temperature, which runs hotter.
• Reading the airport or rooftop wind instead of the wind low at the open slab.
• Finishing while bleed water is still on the surface, sealing in a weak, dusting, scaling skin.
• Waiting for bleed water that never comes on a low-bleed mix while the surface dries and tears.
• Spraying or hosing water onto the surface to keep it wet, raising the skin water to cement ratio.
• Treating an evaporation retarder as a curing compound, so the slab is never actually cured.
• Not staging the fog, retarder, and windbreak before the truck, so there is no time to react.
• Assuming a rich low-water SCM mix is safer in heat when it bleeds less and is more exposed.

Field checklist

Standards and references

ACI 305 is the hot weather concreting guide, and it is where the evaporation rate, the 0.2 lb per square foot per hour action threshold, the nomograph, and the protective measures live. ACI 305.1 is the companion specification, and it carries the concrete temperature limits, where the general discharge limit was set to 95 degrees F in the 2014 edition while many project specs still call out 90 degrees F. Confirm the edition and the number the contract actually adopts before you cite either.

Curing is governed by ACI 308, which covers the methods and durations for holding moisture in the set concrete after finishing, the stage that follows evaporation control. ACI 302 addresses floor and slab construction and finishing, including the finishing practices that interact with bleed water and surface drying. For cold weather, ACI 306 is the parallel guide, a different problem from this one.

On the test-method side, ASTM C1064 is the standard for measuring the temperature of fresh concrete, the concrete-temperature input the rate depends on, and it runs with the slump and air tests at acceptance. ASTM C1579 is the test method for evaluating plastic shrinkage cracking of restrained concrete in the lab, used to compare mixes and fiber additions rather than to run on the jobsite. ACI and ASTM documents change between editions, so verify the year and let the project specification control where it is stricter than these references.

Units, terms, and conversions

The evaporation rate shows up in two unit systems, and the same threshold reads differently across a US spec and a metric one. The customary action level of 0.2 lb per square foot per hour equals 1.0 kg per square meter per hour, a clean conversion worth memorizing because the two numbers are the same line in different clothes. Multiply a metric rate by about 0.205 to read it in customary units.

The inputs carry their own units. Temperature is degrees Fahrenheit on US jobs and Celsius in the formula and metric sources, where 90 degrees F is about 32 degrees C. Wind is miles per hour in the field and kilometers per hour in the equation, where 1 mph is about 1.61 km per hour. Relative humidity is a percentage in conversation and a decimal fraction in the math. Keep the units straight between the meter, the formula, and the spec, because a rate computed in mixed units is a wrong rate.

Evaporation rate

Water leaving the fresh surface per area per time, in lb per square foot per hour or kg per square meter per hour

Plastic shrinkage

Shrinkage of the surface while the concrete is still plastic, driven by evaporation outpacing bleed

Bleed water

Water that rises to the surface as solids settle, which protects the skin until it leaves or is removed

Water to cement ratio (w/c)

Mass of water over mass of cementitious material; low w/c mixes bleed less and crack plastically more easily

SCM

Supplementary cementitious material such as fly ash, slag, or silica fume, which reduces bleed

Evaporation retarder

A sprayed monomolecular film that slows surface water loss during finishing; not a curing compound

Curing compound

A membrane sprayed after finishing to hold moisture in the set concrete during curing, per ACI 308

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