Hot weather concreting and protection field guide

Hot weather concreting, and the water that leaves before it sets

Hot weather concreting is the mirror image of the cold problem. In the cold the reaction slows down and the fight is keeping the concrete from freezing before it sets. In the heat the reaction speeds up and the water leaves, so the concrete stiffens before you can finish it and the surface dries before it can cure. Same material, opposite fight, and the cold-weather playbook does not transfer. The companion cold-weather guide covers the freeze side; this one covers the heat.

Two things happen at once when it gets hot, and both work against you. Cement reacts with water faster as the temperature climbs, so the concrete sets sooner and you have less time to place, consolidate, and finish it. At the same time, hot dry wind and sun pull water off the surface faster than it can rise from inside the mix, so the top dries out while the concrete is still plastic. Lose that surface water before the concrete can cure and you get plastic shrinkage cracking, the number-one hot-weather defect, on a slab that looked fine going down.

The thread through the whole job is water and time. The mix was proportioned to a water-cement ratio, and the heat is trying to take that water back through the surface while shortening the clock you have to work. The discipline is to slow the evaporation, slow the set, keep the concrete cool, and start curing the instant finishing is done. And the one move that wrecks the mix is the obvious one: hosing water onto a stiffening load to make it place easier. That raises the water-cement ratio and lowers the strength, covered later and in the mix-design guide. The water goes on top as a fog and a cure, never into the load.

What counts as hot weather concreting?

Hot weather concreting, under ACI 305, is not a single temperature. It is any combination of high air temperature, low relative humidity, wind, and solar radiation that speeds up moisture loss and hydration enough to hurt the concrete. That is the part crews miss. A 95 F day with calm humid air can be easier to handle than an 80 F day with low humidity and a stiff dry wind, because the wind and the dry air drive the evaporation that does the damage.

ACI 305R is the guide and ACI 305.1 carries the enforceable specification, so know which one your contract points to and which edition. Rather than a hard air-temperature trigger, the controlling condition is the evaporation rate at the surface, which folds the four drivers into one number you can actually act on. The next two sections cover the drivers and that rate. Read the spec for the placement temperature limit and any project-specific triggers, because a job in Phoenix and a job on the Gulf coast hit hot-weather conditions in completely different weather.

The practical read is to plan hot-weather protection on the forecast and the conditions at the slab, not on the calendar month. A spring pour in dry desert air can need every hot-weather measure while a humid summer pour needs fewer. Check the temperature, the humidity, and the wind for the placement window, run the evaporation rate, and stage the protection before the truck rolls.

The four drivers: it is not just air temperature

Four conditions drive hot-weather risk, and air temperature is only one of them. The other three are the temperature of the concrete itself, the relative humidity of the air, and the wind speed across the surface, with solar radiation heating the slab on top of all of it. The reason this matters is that the four combine into the surface evaporation rate, and a moderate air temperature with dry air and wind can produce a worse evaporation rate than a hot, still, humid day.

Each driver has its own lever. Air temperature you cannot change, so you place in the cooler part of the day. Concrete temperature you control by cooling the mix, which is the most direct lever you have, because a cooler mix evaporates and hydrates slower. Humidity you raise locally with fog above the slab. Wind you cut with windbreaks, and sun you cut with shade. The point of naming all four is that fixing only the air temperature, by waiting for a cooler day, does nothing about a dry wind that will still tear the surface.

The driver that surprises people is wind. A 10 to 15 mph dry wind across a fresh slab can multiply the evaporation rate several times over the same conditions with still air, which is why a windbreak is often the cheapest and biggest improvement on a dry pour. The driver that hides is concrete temperature, because a load can batch warm and arrive warmer after a haul in the sun. Measure it, do not assume it.

What is plastic shrinkage cracking?

Plastic shrinkage cracking is the cracking that forms in fresh concrete, while it is still plastic and before it has set, when water evaporates from the surface faster than bleed water can rise to replace it. The surface layer dries and shrinks while the concrete underneath is still full of water, and the top tears into a pattern of short, roughly parallel cracks that can run an inch or two deep. It is the most common hot-weather defect, and it happens in the first hour or two, often before final finishing.

The mechanism is a race between two rates. Bleed water rises to the surface at a certain rate as the heavier solids settle. Evaporation pulls water off the top at another rate set by the four drivers. As long as bleeding keeps up, the surface stays wet. When evaporation outruns bleeding, the surface goes dry and the dried skin shrinks against the wet concrete below, which restrains it, and the surface cracks to relieve the stress. Low-bleed mixes, including those with silica fume or a high fines content, are more prone because they have less bleed water to give.

ACI 305 ties this to a number you can use. The evaporation rate is read off a chart, the ACI nomograph, from the air temperature, the concrete temperature, the relative humidity, and the wind speed. When the evaporation rate approaches about 0.2 lb per square foot per hour, roughly 1.0 kg per square meter per hour, precautions against plastic shrinkage cracking are called for, and on a low-bleed mix the trouble can start lower than that. Treat the 0.2 figure as a widely cited threshold to act on, not a bright line, and hedge it to the edition and the mix. The honest read is that once the chart pushes toward that number, you fog, you cut the wind, and you cover fast, or you crack.

Rapid setting, slump loss, and the shrinking work window

Heat shortens the time you have to work the concrete. Cement hydrates faster as the temperature rises, so a load that gave you an hour of working time at 70 F can give you half that at 95 F, and the slump falls off faster between the truck and the forms. The crew feels it as concrete that goes stiff under the trowel sooner than they expect and a finish that fights back.

The clock starts at the plant, not at the job. ASTM C94 sets an outside limit of 90 minutes or 300 drum revolutions from water meeting the cement to complete discharge, and heat eats that window faster, so a load that sat in a hot yard or a long traffic delay can arrive already part-spent. On a hot pour, sequence the trucks tight so concrete is not waiting in the sun, and check slump and temperature at the point of placement, not at the chute, because the slump that looked fine at the truck can be gone by the time it reaches the back forms.

Slump loss leads straight to the worst hot-weather mistake. A crew watching a load stiffen reaches for the hose to bring the slump back, which raises the water-cement ratio and quietly weakens the concrete. The right answer is a set-retarding admixture and a water reducer designed into the mix, plus a faster placement, covered in the next sections. Fight the clock with chemistry and crew size, not with water.

The strength penalty: fast early, weaker later

Hot concrete gains strength fast early and ends up weaker in the long run. Concrete placed and cured hot reaches its early strength sooner because the reaction runs faster, but the rushed reaction builds a coarser, less uniform structure in the paste, so the ultimate strength comes in lower than the same mix placed cool. You trade a fast 7-day number for a lower 28-day number, and the 28-day number is the one the design was built around.

The figures commonly reported make the trade concrete. Concrete placed around 90 F can show 7-day strengths roughly 10 to 15 percent higher than the same mix placed near 73 F, while the 28-day strength comes in about 5 to 10 percent lower. Curing the first day near 100 F can cost a similar 10 to 15 percent of the 28-day strength. Treat these as typical ranges, not guarantees, because the loss depends on the mix and how hot the concrete actually ran. The direction is what matters: the hotter it cures, the lower it ends up.

This is the long-term cost hiding behind a pour that looked fine. The slab broke its early cylinders high, everyone moved on, and the structure ended up with less margin than the drawings assumed. Keeping the concrete temperature down is about more than working time and cracking. It protects the strength the job is paying for.

Air content in the heat

Heat works against entrained air, which is the concern on any slab that will see freeze-thaw or deicing salts in service. Air-entraining admixtures hold a controlled system of tiny bubbles in the paste, and a hot, stiffening, rapidly hydrating mix is harder to entrain and tends to lose air faster. A producer often has to raise the air-entraining dose in hot weather to land the same air content, and the air can still drop during a long haul, a hot pump line, or over-finishing in the heat.

The result is a durability defect you cannot see going in. A mix that batched at 6 percent can arrive low after a hot haul, and low air on a deicer-exposed slab scales and spalls in the first hard winter. The air-content target and the freeze-thaw decision belong to the mix design, covered in the mix-design guide, but the field has to verify it on a hot pour.

Check air at the point of placement, not just at the truck, by the pressure method under ASTM C231 on normal-weight concrete or the volumetric method under ASTM C173 on lightweight or porous aggregate, run with the slump and temperature at acceptance. On a hot pour for a cold-climate slab, that check is the only thing standing between a thin number on the ticket and a surface that comes apart years later.

What is the maximum temperature for placing concrete in hot weather?

The maximum concrete temperature at placement is set by the specification, commonly held around 90 F and now allowed up to 95 F at discharge under recent ACI 305 editions, with higher temperatures permitted only where data supports it. The number is a ceiling on the concrete itself, measured per ASTM C1064, not on the air. A load over the limit stiffens fast, loses slump and working time, and gives up long-term strength, which is why the limit exists.

Know which document and edition control. ACI 305.1-14 raised the long-standing 90 F discharge limit to 95 F, and the standard allows placement above that when the producer can show with data that the concrete will still perform. Some specifications hold a tighter limit for a specific element, and some jobs set a lower number for mass concrete to control the heat. Read the spec for the number that governs your pour, and do not assume the 95 F figure applies if the contract says otherwise.

What to do at the chute when a load reads over the limit is a real decision, not a formality. The honest moves are to reject the load and have the plant cool the next one, many can drop a load 5 to 10 F with ice, or get written approval from the engineer of record before placing. What you do not do is place a hot load quietly and hope. Measure the concrete temperature at acceptance, every load on a hot pour, and treat the limit as the limit.

How do you cool concrete in hot weather?

You cool the mix by cooling its ingredients, and the order runs from the easiest lever to the most expensive. Chilled mixing water comes first because water is simple to cool and has a high heat capacity, so dropping the water temperature pulls the whole batch down for the least trouble. Replacing part of the mixing water with crushed or flaked ice goes further, because the ice absorbs a large amount of heat as it melts, and the ice has to be fully melted before the concrete is discharged so it does not leave voids.

When water and ice cannot carry it, cool the aggregate, which is the largest part of the mix by weight and the largest store of heat. Plants shade aggregate stockpiles, sprinkle and evaporatively cool the coarse aggregate, or chill it, and a few degrees off the aggregate moves the batch more than the same effort on a smaller ingredient. Shading and sprinkling the stockpiles is the low-cost version that any plant can do on a hot week.

For the heavy cases, liquid nitrogen cools mixed concrete directly. Injected into the mixing water or the fresh concrete, it drops the temperature fast and far, and it is the tool reached for on mass placements and tight temperature specs where ice alone cannot hit the number. It costs real money, so it shows up on large mat pours and critical elements, not ordinary flatwork. The rule across all of these is that cooling is the plant's job arranged ahead of the pour, and the field's job is to confirm the concrete arrived under the limit by measuring it per ASTM C1064 at the chute.

Retarders, water reducers, and the right way to buy time

The way to fight slump loss and a short work window is chemistry in the approved mix, not water at the chute. A set-retarding admixture, ASTM C494 Type B or the water-reducing-and-retarding Type D, slows the hydration so the concrete stays workable longer, which buys back the time the heat took. On long hauls and big hot pours, the retarder is what keeps the back forms placeable while the crew finishes the front.

A water reducer raises the slump without adding water. Type A reduces the water needed for a given slump, and the Type F and Type G high-range water reducers, the superplasticizers, turn a stiff low-water mix into a flowing one with the water-cement ratio held where the design set it. This is the move when a load comes in stiff: the slump comes up from admixture, the strength and durability hold, and nobody touched the hose. The mix-design guide covers how these fit the proportions.

Hydration-stabilizing admixtures go further on extreme hauls and hot stretches, holding the concrete in a controlled, near-dormant state and then releasing it to set, which can extend the usable life of a load well past the normal window. Whatever the product, it is part of the approved mix design and dosed at the plant, not a field decision at the chute. A retarder dumped in on site changes the set in ways nobody planned and nobody recorded, and an overdose can leave a slab that will not finish for hours.

Can you add water to concrete in hot weather?

No, not to fix slump. Adding water to a stiffening load raises the water-cement ratio above what the mix was designed to, and that lowers the strength and raises the permeability even though the concrete places easier. It is the single most common and most damaging hot-weather mistake, because the slump comes up, the crew is satisfied, and the cylinders break low a month later when nobody is connecting the two.

The only water that may go in is the design water the plant withheld, added once and recorded on the batch ticket, within the water-cement ratio the mix was proportioned to. Anything past that is over the limit. When a load needs more flow than the withheld water gives, the answer is a water reducer, which raises the slump with no added water, covered above and in the mix-design guide. Same workability on the cone, opposite outcome in the slab.

This is where the foreman has to hold the line against the crew that just wants to place the load. The water that fixes the surface in hot weather goes on top, as a fog to slow evaporation and as curing water after finishing. It never goes into the mix. An undocumented gallon hosed into the truck is the reason a strength problem can never be explained later, so if water goes in, it is design water and it is written down.

Timing the pour: early morning and night placement

The cheapest hot-weather measure is choosing when to pour. Place in the early morning or at night and you avoid the worst of the air temperature, the solar load on the slab, and often the daytime wind, so the concrete goes in cooler and the evaporation rate at the surface is lower for free. On a brutal week, moving the pour off mid-day does more than any single admixture.

Night placement has its own trade-offs to plan for. Lighting has to be set so the crew can finish properly in the dark, and the temperature can swing as the night cools, which changes the set time across a long pour. A slab placed late evening into early morning can hit its finishing window in the coolest, calmest hours, which is exactly what you want, but the schedule and the crew have to be built around it rather than treated as overtime on a normal day.

Mid-day summer placement is the pour to avoid when you can. Peak air temperature, full sun on the slab, and a dry afternoon wind stack all four drivers at once, and that is when plastic shrinkage cracking and flash setting show up together. If the schedule forces a mid-day pour, that is the one that needs every other measure in this guide running at full strength: cooled concrete, retarder, fog, windbreaks, shade, and a finishing crew ready to move.

Cooling and dampening the subgrade and forms

A hot, dry subgrade and hot formwork steal water from the bottom and sides of the concrete the same way the sun steals it from the top. Dry sand or a baked subgrade under a slab pulls water out of the fresh concrete by suction, and steel forms or rebar sitting in the sun act as hot surfaces that dry and stiffen the concrete at the contact face. Place on a dry hot subgrade and the bottom of the slab loses water it needed to cure.

Dampen the subgrade ahead of the pour so it is moist but not muddy, with no standing water. The target is a subgrade that will not drink from the concrete, which means wetting it down the evening before or early enough that the surface water has soaked in by placement. Standing water in the forms is the opposite problem: it adds to the mix water at the bottom of the slab and raises the water-cement ratio right where you cannot see it. Moist, not flooded, is the line.

Cool and shade the forms and the steel where you can. Hose down hot formwork and keep reinforcing out of direct sun before placement, or place when the sun is off them, because a rebar mat baking at well over 120 F in summer sun is a heat source dropped into the middle of the pour. Knocking the temperature off the subgrade, the forms, and the steel before the concrete arrives is the same idea as cooling the mix: take the heat out before it can do damage, not after.

Evaporation retarder: the film between finishing passes

An evaporation retarder is a spray-on film that slows surface drying while the concrete is still plastic, applied between finishing passes. It is an emulsion of a fatty alcohol, often cetyl alcohol or a similar monomolecular film former, that you spray onto the fresh surface to lay down a single-molecule layer over the bleed water. That film cuts evaporation sharply, reported to reduce moisture loss on the order of 80 percent in wind and 40 percent in direct sun, which holds the surface wet long enough to finish without it crusting and cracking.

The point to get right is that this is not a curing compound. The evaporation retarder protects the surface during the plastic stage, between bull-floating and final troweling, and the film breaks up and disappears as the concrete sets and you work it. It buys finishing time and fends off plastic shrinkage cracking, but it does nothing for curing once the concrete has set. After finishing, you still cure, covered below.

Use it the way the trade does, as a thin spray between passes, not as a finishing aid worked into the surface. Spraying it on and then troweling the diluted film into the concrete adds water to the surface paste and leaves a weak, dusting skin, which is the rookie misuse. Lay it on to protect the surface, let it do its job between passes, and never trowel it in. On a dry windy slab it is one of the better tools you have, alongside fog and a windbreak.

Fogging: raising the humidity above the slab

Fogging is spraying a fine mist of water into the air just above the fresh concrete to raise the local humidity and cool the air, which slows the evaporation rate at the surface. The trick is in the word fog. You are putting water into the air, not onto the concrete, so the mist has to be fine enough to hang above the slab and not land as droplets that mar the surface or add to the mix water. A proper fog nozzle, not a garden-hose spray, is the tool.

Fogging attacks two of the four drivers at once. It raises the relative humidity in the layer of air right over the slab and it cools that air, both of which drop the evaporation rate read off the ACI chart. On a dry, low-humidity day it is one of the few ways to fight the humidity driver directly, and it runs from placement through finishing whenever the evaporation rate is pushing the threshold.

Keep the water off the surface. A coarse spray that puts standing water on the bleed water raises the surface water-cement ratio and leaves a weak skin, the same defect as troweling water in or over-using an evaporation retarder. The honest version is a fine mist held above the slab between and during finishing operations, kept up until curing takes over. Done right, fogging and an evaporation retarder together keep a dry windy slab wet long enough to finish and cure.

Windbreaks and sunshades at the surface

Cutting the wind and the sun at the surface attacks the two drivers you can physically block. Wind is often the biggest single contributor to a high evaporation rate, so a windbreak, a temporary barrier of fencing fabric, tarps, or panels upwind of the slab, can drop the evaporation rate more than any admixture. On an open site with a steady dry wind, the windbreak is the cheapest large improvement available, and it is the measure most often skipped because nobody wants to build it.

Sunshades cut the solar load that heats the slab surface and raises the concrete temperature locally. Shade over the placement keeps the sun off the fresh concrete during finishing, lowering the surface temperature and the evaporation that rides on it. On a large flat pour you cannot shade everything, but shading the active finishing area and the staged materials helps where it counts.

These are temporary and they go up before the pour, not after the surface is already drying. The combination that holds a dry hot slab is a windbreak to kill the wind, shade to cut the sun, fog to raise the humidity, and an evaporation retarder on the surface, with cooled concrete underneath. No single one of them carries a bad day alone. Stacked together they bring the evaporation rate back under the threshold.

Finishing fast: the crew, the tools, the plan

The hot-weather finishing window is short, so the crew and the plan have to be ready before the concrete arrives. Heat brings the set forward, which means bleeding finishes sooner and the surface is ready for each operation earlier than the crew expects. Be staffed and tooled to keep up, because a slab that gets ahead of an undersized crew cracks and crusts while they are still catching up on the front of it.

The rookie mistake cuts the other way too. New finishers, used to waiting for bleed water to clear, can trowel a hot slab too early or seal the surface while there is still water under it, trapping it and leaving a weak, dusting top. The skill in the heat is reading the surface fast and matching each operation to it, not running the same clock as a 70 F day. The bleed water disappears faster, the set comes sooner, and both move on the conditions.

Plan the sequence and oversize the crew for a hot pour. Stage the bull floats, the edgers, and the trowels so the crew moves with the set instead of chasing it, keep the placement rate matched to what the finishers can actually keep up with, and have the fog and evaporation retarder running so the surface holds while they work. The pour that fails in the heat is usually the one where the crew was sized for a mild day and the slab set faster than they could finish it.

Curing immediately and continuously in the heat

Start curing the instant finishing is done, with no gap, because in the heat the surface dries the moment it is left alone. Curing in hot weather is about keeping the concrete wet and cool while it gains strength, and the enemy is the same evaporation that caused plastic cracking, now working on a setting surface that needs its water to hydrate. A delay of even a short time between final finishing and the start of curing, on a hot dry day, is enough to dry the top and cost surface strength.

Water curing is the strongest method in the heat because it cools as it wets. Wet burlap kept continuously damp, soaker hoses, or ponding hold water against the surface and pull heat out as it evaporates, which is exactly what a hot slab needs. The hard part is continuous. Burlap that dries out and then gets re-wet puts the surface through wet-dry cycles that can do their own damage, so once water curing starts it stays wet for the full curing period, commonly seven days for ordinary mixes and longer where the spec calls for it.

Curing compounds work where water curing is not practical, but apply them right and apply them fast. A membrane-forming curing compound under ASTM C309 sprayed at the proper rate seals the moisture in, and on a hot pour it goes on as soon as the surface will take it and the bleed sheen is gone, before the surface can dry. A white-pigmented compound also reflects sun and keeps the slab cooler. The deeper curing-by-method picture lives in the curing guide; the hot-weather rule is simple. Cure now, cure wet where you can, and never leave a finished hot slab uncovered.

Cold joints on a big summer pour

A cold joint is the weak, visible plane that forms when fresh concrete is placed against concrete that has already started to set, so the two do not knit together. Heat makes cold joints easy to create, because the first concrete down sets faster and the working window between lifts or between trucks is shorter. On a big summer pour, the placing rate has to stay ahead of the set, or the seam between what is placed and what is coming sets up before they can be vibrated together.

The defense is sequence, rate, and retarder. Plan the placement so each new concrete lands against concrete that is still plastic and can be consolidated into it, keep enough trucks coming that the pour does not stall in the heat, and use the set-retarding admixture to lengthen the window so the previous lift is still workable when the next arrives. A pour that stalls waiting on trucks in the sun is a pour growing a cold joint at the leading edge.

On a large slab or wall this is a planning problem before it is a placing problem. Lay out the pour so the leading edge keeps moving, size the supply to the conditions and not to a mild-day rate, and on a critical element get the retarder dose set for the haul and the heat. The cold joint you can see is a finish problem. The one that hides inside a wall is a structural one, and the heat is what creates it when the rate falls behind the set.

Mass concrete and heat in the summer

Mass concrete in hot weather stacks two heat problems on top of each other. A thick footing, a mat, or a heavy wall generates its own heat as the cement hydrates and cannot shed it fast, so the core runs hot regardless of the weather, and a hot-weather placement starts that mass off warmer still. The combination drives up the peak core temperature and the difference between the hot core and the cooler surface, and that difference is what cracks mass concrete from thermal stress.

The mix and the placement carry a thermal-control plan. The producer cuts the heat of hydration with high replacement of cement by slag or fly ash and sometimes a low-heat cement, which also slows the strength gain, so the spec often judges the mix at a later age, commonly 56 or 90 days, instead of penalizing it at 28. The placement starts cool, which is where ice or liquid nitrogen earns its cost, and the plan sets limits on the peak core temperature and the core-to-surface difference. The cold-weather guide covers the same thermal-difference mechanism from the other direction.

The field job is to start cool and control the cooldown. Place the mass concrete as cool as the spec allows, monitor the core and surface temperatures against the plan's limits, and protect the surface so it does not cool too fast relative to the core, which would open the same cracks as a cold placement. The thermal-control plan and the engineer of record govern the numbers, alongside ACI 305 and the mass-concrete provisions of ACI 301.

Preventing and fixing plastic shrinkage cracks

Plastic shrinkage cracks are prevented by the combination, not by any one measure, because they come from an evaporation rate that beats the bleed rate. Drop the evaporation rate below the threshold and the cracks do not form. That means stacking the controls on a dry hot day: cooled concrete to lower the surface temperature, a windbreak to kill the wind, shade to cut the sun, fog to raise the humidity, and an evaporation retarder on the surface between finishing passes, with curing started the instant finishing ends.

The order to think about it is by driver. Run the evaporation rate off the ACI chart, see which drivers are pushing it up, and attack those. A dry windy day is a wind and humidity problem, so windbreak and fog come first. A still, sunny, hot day is a temperature and sun problem, so cooler concrete and shade lead. The point is to bring the number under about 0.2 lb per square foot per hour by whatever combination the conditions demand, and to keep it there until curing takes over.

Once the cracks have formed while the concrete is still plastic, there is a narrow window to close them. Re-working the surface, re-floating or re-troweling the cracked area before the concrete has set, can close plastic shrinkage cracks that are caught early, while fog and an evaporation retarder hold the surface. Miss that window and the cracks are in the hardened concrete, where the honest options are sealing them or living with them, and a deep pattern over a structural slab becomes a durability question for the engineer. Prevention is far cheaper than the fix, which is why the controls go up before the pour.

Testing and making cylinders in the heat

The fresh tests run the same in the heat as any pour, but the temperature reading carries more weight and the cylinders need protecting from the conditions. Take slump under ASTM C143, air by ASTM C231 or C173, and temperature by ASTM C1064 at the point of placement, and on a hot pour the temperature is the one that can reject a load, so it gets measured on every load against the placement limit, not spot-checked.

Making cylinders in the heat has its own trap. ASTM C31 requires field-cast acceptance cylinders to start their initial cure in a controlled temperature range, commonly cited as 60 to 80 F for ordinary-strength concrete, for the first day before they go to the lab. Leave them sitting on the slab in the summer sun and they cure hot, gain fast, and break low later, so the cylinders read worse than the concrete actually is and you have manufactured a strength problem on the test bench. Cure boxes, a cooler with temperature control, or a shaded conditioned space keep the initial cure in range.

The detailed make, cure, and break procedure lives in the cylinder-testing guide; the hot-weather rule is that the cylinders have to be protected from the heat the same as the slab. A common summer failure is acceptance cylinders cooked on site that fail an otherwise sound pour, which then drags everyone into an investigation of concrete that was fine. Protect the cylinders, log the concrete temperature on every load, and the test data defends the pour instead of indicting it.

The large mat pour in summer

A large mat placement in summer is where every hot-weather measure runs at once. The mat or raft under a generator yard, a switchgear lineup, or rows of loaded equipment is a high-volume continuous pour that is both mass concrete and a long-duration placement, often run overnight to beat the heat, and it stacks the cold-joint risk, the thermal-control problem, and the evaporation problem into one job.

The plan is built around heat and rate. The mix uses high SCM replacement to cut the heat of hydration and is judged at a later age, the concrete is cooled with ice or liquid nitrogen to start under the temperature limit and inside the thermal plan, and the trucks are sequenced to hold the placing rate ahead of the set so the leading edge stays plastic and no cold joint forms across the mat. Retarder keeps the window open across the long pour, and core and surface temperatures are monitored against the thermal-control limits.

On a job like this several pours run with their own rules: the mass mat, the structural slabs and walls, the equipment pads, and the grout under base plates, each with its own mix and acceptance age. The recurring failures are a cold joint where the rate fell behind, plastic cracking on an exposed surface that got ahead of the fog, and cylinders cooked in the summer night air. Plan the rate, the cooling, the surface protection, and the cylinder curing together, because on a large summer mat they all come due at once.

Planning the hot pour

Hot-weather work is won before the truck arrives, the same as the cold pour from the other guide. You cannot improvise protection while the surface is drying, because the window is too short and the materials are not on the truck. The cooling, the retarder, the fog rig, the windbreaks and shade, the curing materials, and the cylinder cure box all have to be staged and tested before the first yard hits the forms.

Build the plan on the conditions for the placement window, not the calendar. Run the evaporation rate for the forecast temperature, humidity, and wind, and decide which drivers you are fighting. Coordinate the chilled water, ice, or aggregate cooling and any retarder and water reducer with the plant so the concrete arrives under the placement limit with the working time the haul needs. Stage the fog nozzles and water, the windbreaks and shade, the evaporation retarder, and the curing materials so the surface is protected from finishing straight into curing with no gap. Oversize the finishing crew for the short window, and sequence the trucks so a load is not baking in the sun.

Then keep the contingency and be willing to use it. If the heat runs worse than forecast or the cooling cannot hold the limit, you need a fallback decided ahead of time: move the pour to night, add cooling, or wait for a better window. The honest superintendent move is the same on a hot day as a cold one. A pour that cannot be cooled and protected is a pour that waits, and that call is cheaper than a cracked, weak slab and the repair that follows. Decide it on the conditions and the staged protection, and write down what drove the call.

What to document

On a summer pour, a cracked surface or a cylinder that breaks low sends everyone back to the records, and what is written there is the only account of how hot the concrete ran and whether the evaporation controls kept up. The record is what shows the concrete arrived under the placement limit, the evaporation rate was controlled, and the surface was protected and cured without a gap. Write it through the pour against the clock, not from memory after the crew has gone home.

The table is the spine of a hot-weather record for a placement. Tie it to the batch ticket and the location, and keep it next to the slump, air, and strength records for the same pour, because the inspector reading it later wants to see that the temperature limit was met, the evaporation controls were running, and no water went into the load.

Common mistakes

• Adding water to a stiffening load to fix slump, raising the water-cement ratio and lowering strength.
• Running no evaporation control on a dry windy slab, so the surface plastic-shrinkage cracks before it sets.
• Pouring mid-day in summer when timing to early morning or night would have done most of the work for free.
• Leaving the finished slab uncovered, so curing starts late and the hot surface dries before it can gain strength.
• Placing on a dry hot subgrade or against sun-baked forms and rebar that pull water out of the concrete.
• Skipping the set-retarding admixture on a big summer pour, so the leading edge sets and grows a cold joint.
• Treating only the air temperature and ignoring the wind and humidity that drive the evaporation rate.
• Troweling an evaporation retarder or fog water into the surface, leaving a weak, dusting skin.
• Leaving acceptance cylinders in the summer sun, so they cure hot and break low on a sound pour.

Field checklist

Standards and references

ACI 305 is the guide to hot weather concreting, and it is where the four-driver definition, the evaporation-rate chart and the roughly 0.2 lb per square foot per hour precaution threshold, the placement-temperature limit, and the cooling and protection measures live. ACI 305R is the guide and ACI 305.1 carries the enforceable specification, so confirm which one the contract adopts and which edition, because the placement-temperature limit moved from 90 F to 95 F at discharge in ACI 305.1-14 and the provisions shift between editions. The cold-weather companion, ACI 306, is the opposite problem covered in the cold-weather guide.

Strength and durability live on the design side. ACI 318, the structural concrete code, and ACI 301, the specification for structural concrete, set the required strengths, the durability and exposure requirements, the acceptance criteria, and the mass-concrete provisions the engineer of record works from. The mix design, the water-cement ratio, and the exposure classes are covered in the mix-design guide, and the strength penalty from hot curing is a performance fact, not a code limit, so treat the 5 to 10 percent figures as typical and let the spec set what it requires.

On the test-method side, ASTM C1064 measures the temperature of fresh concrete, ASTM C143 measures slump, and ASTM C231 and C173 measure air content by the pressure and volumetric methods, all at acceptance. ASTM C31 covers making and the initial cure of the field cylinders, with the controlled initial-cure temperature range that matters in the heat, and ASTM C39 breaks them. ASTM C94 sets the 90-minute or 300-revolution discharge limit, ASTM C494 covers the retarding and water-reducing admixtures, and ASTM C309 covers membrane-forming curing compounds. ACI and ASTM documents change between editions, so verify the year and let the project specification and the engineer of record control where they are stricter than these references.

Units, terms, and conversions

Hot-weather concrete numbers read in US and metric units across an international spec, so the same limit can look different on a drawing, a ticket, and a product sheet. The evaporation-rate threshold of about 0.2 lb per square foot per hour is roughly 1.0 kg per square meter per hour. The 95 F placement limit is about 35 C, and 90 F is about 32 C. Temperatures read in Fahrenheit on US jobs and Celsius elsewhere, where a change of 1 C is 1.8 F.

Keep the terms straight, because the same word gets used loosely on a hot site. The evaporation rate is the water leaving the surface per area per time, the thing the ACI chart predicts, not the same as the bleed rate, which is the water rising from inside. An evaporation retarder protects the plastic surface between finishing passes and is not a curing compound, which seals moisture in after the concrete sets. A set retarder buys working time by slowing hydration, while a water reducer raises slump with no added water. Confirm units and terms between the spec, the ticket, and the product, because a limit written one way and read another is a dispute waiting to happen.

Hot weather concreting

Placing and protecting concrete when high air temperature, low humidity, wind, and sun speed up evaporation and hydration, per ACI 305

Evaporation rate

The water leaving the fresh surface per area per time, read from the ACI chart; precautions apply near 0.2 lb/sq ft/hr

Plastic shrinkage cracking

Cracks in fresh concrete when surface evaporation outruns the bleed rate, drying and shrinking the top before it sets

Bleed rate

The rate at which water rises to the surface as solids settle; when evaporation beats it, the surface cracks

Evaporation retarder

A monomolecular film sprayed on the plastic surface to slow drying between finishing passes; not a curing compound

Set retarder

An admixture (ASTM C494 Type B or D) that slows hydration to extend working time in the heat

Fogging

A fine mist held above the slab to raise local humidity and cool the air, slowing surface evaporation

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