INTRODUCTIONWhile hot weather conditions are commonlyencountered in summer, combinations of hightemperatures, winds and low humidity could resultin conditions leading to problems with concreteplacement and finishing at any time.

This data sheet provides guidance on the effectsof hot weather conditions on the properties ofconcrete, and the precautions that should be taken,particularly with flatwork, to minimize any potentialadverse effects when placing concrete under theseconditions.

WHAT IS HOT WEATHER?For the purposes of this data sheet, hot weather isany combination of:■ high ambient temperature;■ low relative humidity;■ high wind speed.

AS 13791 places a 35°C limit on the maximumconcrete temperature at the time of delivery.However, when the air temperature rises above30°C, it is usually recommended that precautionsbe taken, particularly if there is also hot dry wind.

This is firstly to ensure an acceptable concretetemperature at the point of delivery, and secondly toavoid problems with plastic shrinkage cracking andpremature stiffening of the concrete.

Figure 1 can be used to estimate the possibilityof plastic shrinkage cracking occurring, and hencethe need for suitable precautions to be taken. Thisnomograph is used to combine the effects of airtemperature, relative humidity, concrete temperatureand wind speed, to estimate the rate at which waterwill evaporate from the surface of the concrete. As ageneral rule, if the rate of evaporation is greaterthan 1 kg of water per square metre of concrete perhour (1 kg/m2/h), then precautions againstpremature drying and plastic shrinkage crackingshould be taken.

Alternatively, the rate of evaporation can becalculated from the following equation:

E = 5([Tc + 18]2.5 – r[Ta + 18]2.5)(V + 4) x 10-6

whereE = evaporation rate, kg/m2/hr = relative humidity/100Ta = air temperature, °CTc = concrete (water surface) temperature, °CV = wind velocity, km/h.

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HOT-WEATHER Concreting>

COMBINATIONS of high temperatures,

winds and low humidity could result in

conditions leading to problems with

concrete placement and finishing.

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AIR TEMPERATURE (°C)

USE OF CHART:

1

NOTE: Evaporation rates approaching1.0 kg/m2/h are likely to necessitateprecautions against premature drying

From air temperaturemove UP to relative humidity.

2 Move RIGHT to concrete temperature.

3 Move DOWN to wind speed.

4 Move LEFT to read rate of evaporation.

RATE OF WATER EVAPORATION (kg/m2/h)

5 10 15 20 25 30 35

0.5

1.0

1.5

2.0

0

2.5

3.0

3.5

4.0

4.5

5

10

15

20

25

30

35

5

0

10

15

20

25

30

WIND SPEED (km/h) = 40

35

40 = CONCRETE TEMPERATURE (°C)

70

RELATIVE HUMIDITY (%) = 100

90

80

60

50

40

30

20

10

Figure 1: Effect of concrete and air temperatures, relative humidity and wind velocity on the rate ofevaporation of surface moisture from concrete (after ACI 3052)

INITIAL SETTING TIME (hours)

AMBIENT AIR TEMPERATURE (°C)5

1

2

4

6

3

5

7

01510 20 25 30 35

8

Figure 2: Influence of air temperature on settingtimes of concrete made with Type GP cement

SLUMP (mm)

FRESH CONCRETE TEMPERATURE (°C)0

20

40

60

80

100

010 20 30 40 50

120

Figure 3: Decrease in workability of fresh concrete(as measured by slump), made with constant watercontent, as temperature increases

EFFECTS OF HOT WEATHER CONDITIONSMost of the problems associated with placingconcrete in hot weather conditions relate to theincreased rate of cement hydration at highertemperatures and the increased rate of evaporationof moisture from the fresh concrete.

The properties of concrete that may be affectedby hot weather conditions include:

■ Setting time As the concrete temperatureincreases, the setting time, and thus the time toplace, compact and finish the concrete isreduced Figure 2.

■ Workability and slump Higher temperaturesreduce the workability (or slump) of theconcrete more rapidly with time Figure 3.Adding more water to improve the workability ofthe mix decreases the strength and increasesthe permeability, and ultimately affects thedurability of the concrete.

■ Compressive strength Higher water demandand higher concrete temperature could lead toreduced 28-day strengths. If more water is addedto the concrete mix at higher temperatures tomaintain or restore workability, the water-cement ratio will be increased, resulting in aloss of both potential strength and durability.This may also increase the drying shrinkage ofthe hardened concrete. Where water is notadded, the reduced setting time and workabilityincrease the potential for inadequate compaction(itself of a major influence on strength), theformation of cold joints and poor finishes.

■ Concrete temperature Hot weather conditionsmay accentuate the temperature rise in concretecaused by the heat of hydration. In large sectionsthermal gradients through the element maycause thermal cracking. Laboratory tests showthat sustained higher temperatures significantlyinfluence the compressive strength gain ofhardened concrete Figure 4. While increasedconcrete temperatures may result in an increasein the early rate of strength gain, in the longerterm, concrete cured at lower temperatures willachieve higher ultimate strength.

■ Poor surface appearance With the increasedrate of evaporation, the surface of the concretewill dry out and stiffen. In the case of flatworkthis may lead to premature finishing of thesurface, trapping an amount of bleed waterwithin the mix. The compacted surface layer(from finishing) may cause the rising bleed waterto be trapped below the surface, resulting indebonding of the surface layer and subsequentflaking. Also, colour differences on the surfacemay result from different rates of hydration andcooling effects.

■ Plastic shrinkage cracking Hot weatherconditions accelerate the loss of moisture fromthe surface. If the rate of evaporation is greaterthan the rate of bleeding (rate at which waterrises to the surface), surface drying will occur,resulting in shrinkage of the concrete. When theshrinkage stresses exceed the tensile capacityof the concrete, cracking will occur. Thelikelihood of plastic shrinkage cracking istherefore greater whenever hot weatherconditions increase evaporation or the concretehas a reduced bleeding rate. Plastic shrinkagecracks can be quite deep, as the plastic concretehas little capacity to resist shrinkage stresses,and cracks continue to widen and propagateuntil the shrinkage stresses are relieved. (Noteplastic shrinkage cracks seldom extend to freeedges, as unrestrained contraction of theconcrete is possible at these locations.)

■ Thermal cracking Concrete is at risk of thermalcracking when it is first placed, and the heat ofhydration raises the temperature of the interiorof the concrete. Rapid changes in thetemperature of the external concrete surface,such as when concrete slabs, walls or pavementsare placed on a hot day followed by a cool night,lead to thermal gradients between the warm/hotinterior and the colder external surface. Thewarmer interior provides a restraint to thecolder external surface, which wants to contract.

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% OF 28-DAY COMPRESSIVE STRENGTH OF CONCRETE CURED AT 23°C

AGE (days)

30°C

40°C

50°C

23°C

Specimens moist-cured at temperature indicated,for first 28 days, then moist-cured at 23°C thereafter

1

20

40

80

120

60

100

03 7 28 90 365

140

Figure 4: Effect of high curing temperatures onconcrete compressive strength

Depending on the temperature differential,cracking of the concrete may result. Massive orthick concrete elements are more at riskbecause of the insulating effect that the concreteprovides to the interior of the element.

MINIMISING THE EFFECTS OF HOT WEATHERCONDITIONS■ Control concrete temperature AS 1379 requires

that concrete temperatures at the point ofdelivery be within the range 5°C to 35°C. Forhigh ambient temperatures, precautions need tobe taken by the supplier to ensure that theconcrete temperature at the point of delivery iswithin the allowable range.There are a number of options to control thetemperature of concrete, including adjusting thetemperature of the ingredients and/or cooling ofthe concrete mix. The sensitivity of thetemperature of a normal concrete mixture tothat of its components can be demonstrated bythe following formula:

T = 0.1 Tc + 0.6Ta + 0.3Tw

whereT = concrete temperatureTc = temperature of the cementTa = temperature of the aggregateTw = temperature of the water.

Because aggregates make up the bulk of theconcrete, and also have the highest heatcapacity, they have the greatest effect on thetemperature of the freshly mixed concrete.Unfortunately the temperature of theaggregates is also the most difficult to control.Some benefit can be gained from shadingstockpiles from the sun and/or keeping themmoist with sprinklers. Storage in bins (paintedwhite) will also assist. The mix water offers the most potential fortemperature reduction, particularly by addingcrushed ice to it, as the latent heat of the ice isconsiderably higher than that of water.The temperature of the cement does not usuallycontribute significantly to the temperature offreshly mixed concrete because of its low specificheat and relatively small mass in the mix.Liquid nitrogen, injected into the concrete whilemixing, may also be utilised. Latent thermalenergy on vaporisation to gas cools the concretedramatically without any known deleteriouseffect. It should be noted that this process isusually economical only on major projectsinvolving construction of large concrete elements.

■ Admixtures Various types of chemicaladmixtures can be beneficial in hot weatherconditions. Water reducers (plasticisers) can beused to reduce the water content or to aid theworkability. This enables rapid placement andconsolidation of the concrete with beneficialeffects on the ultimate strength and durability. Set-retarders can provide additional time toplace and finish flatwork. With rapid drying ofthe surface, caution is required with the use ofset retarders, as the surface may appear readyfor finishing, but the concrete below may still beplastic from the retarder, leading to a ‘spongy’feel under foot. This could affect the uniformityof the surface finish.

■ Cement type Selection of a particular cementtype may provide additional benefits. Usingslower hydration cements (eg Type LH) withlower rate of heat development can provideextra time for placing and finishing, reduce theconcrete temperature and the risk of thermalcracking upon cooling of the concrete.

■ Cement content The temperature increasefrom hydration of cement in a given concrete isproportional to its cement content. The cementcontent therefore should be limited to thatrequired to provide strength and durability.

PRECAUTIONS IN HOT WEATHER CONDITIONS■ General Problems usually arise when site

personnel are not aware of the effect of weatherconditions and/or weather conditions changeduring the placing and/or finishing of theconcrete. Improvised responses to unexpectedchanges is not recommended as the damagethat can result from uncontrolled effects of hotweather conditions may never be completelyalleviated. The first option to be considered in hot, adverseweather conditions is whether or not topostpone the placement of concrete. It is oftenbetter to wait than risk costly repairs (or evenreplacement) of defective work and dissatisfiedclients seeking compensation. If work is toproceed, proper planning from careful selectionof materials to procedures for hot weather workis essential if the associated risks are to beminimised.

■ Planning A successful, well-run, hot-weatherproject is the result of thorough and carefulplanning. Planning for hot weather conditions isessential because of the potential effects onfresh and recently placed concrete. For plastic

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concrete these include increased water demandand risk of plastic shrinkage cracking, greaterslump loss, faster setting and difficulty incontrolling entrained air content; for hardenedconcrete they include reduced strength anddurability, and increased drying shrinkage.While the designer or specifier may controlsome items on the following list, it may notalways be possible to anticipate just when andunder what weather conditions the work will becarried out. Therefore the builders and sub-contractors should also be aware/alert toall of the possibilities.■ Where hot weather conditions are likely to

present a problem, consult the concretesupplier as early as possible.

■ Have standby equipment and manpower forall stages.

■ Use the largest size and amount of coarseaggregate compatible with the job. This alsohelps minimise the tendency of the concreteto crack.

■ In determining the slumps of the concrete tobe used, consider the effects of hot weatheron the ability to place and finish the concrete.

■ Programme concreting for the cooler partsof the day, or even schedule night-timeplacement if possible.

■ Specify the maximum acceptable deliverytemperature of the concrete so that thesupplier can plan to cool the materials asneeded.

■ Avoid delays at all stages.■ Plan the locations of construction joints

ahead of time with hot weather contingenciesin mind.

■ Consider spacing contraction (control) jointsat slightly smaller intervals than whenconcreting at lower temperatures.

■ Use sunshades or windbreaks.■ Delay construction of indoor slabs on grade

until the walls are up and the roof is on.■ Pay attention to the rate of concrete

placement. Be ready to notify the concretesupplier promptly of any changes in schedule,which may become needed as the jobprogresses.

■ Keep an evaporative retarder (aliphaticalcohol) on site in case conditions require its use.

■ Concrete Production The concrete supplier’sresponsibility is to manufacture and deliver theconcrete in accordance with AS 1379. Thefollowing methods can be used by the supplier toassist with the placing of concrete in hot weather:■ Shade stockpiles, sprinkle aggregates ahead

of time for evaporative cooling, or cool themby other means.

■ Paint the mixer and storage bins white tominimise absorption of heat from the sun.

■ Use ice as part of the mix water or cool theconcrete with liquid nitrogen.

■ Delivery and Discharge Delays in delivery canundo the best mixing practices. The concretesupplier should set up and maintain a gooddelay-free schedule for delivering the concreteto the required location on the site.■ Minimise transport time and avoid

unnecessary delays.■ Avoid prolonged mixing. Transit mixer trucks

should be discharged as soon as possibleafter the water has been added to the mix.

■ For large projects, consider batching andmixing the materials using a job-site plant.

■ Water should not be added to pre-mixedconcrete at the job site unless it is part of theamount required initially for the specifiedmaximum water-cement ratio and thespecified slump.

■ Placing and Finishing For successful placingand finishing it is necessary to provide anenvironment in which workers and equipmentcan function well, and concrete can be adequatelyprotected from rapid warming and/or drying. ■ Schedule placement for the cool time of the

day such as early morning or late afternoon.On some jobs, concreting at night (ifpermitted) may be more advantageous.

■ Have all forms, equipment and workersready to receive and handle concrete,especially the first delivery.

■ Use sunshades and/or windbreaks.■ Keep all equipment that touches the

concrete cool (chutes, conveyors, pumplines, tremies, reinforcement and buggies).Protecting equipment from the direct sunwill assist. If it can’t be kept continuouslycool, spray-cool it as necessary with water.

■ For a slab without a vapour barrier under it,dampen the sub-grade before placingconcrete.

■ Use cool water to dampen side forms forslabs or walls.

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■ Use a thermometer to monitor thetemperature at which concrete is beingdelivered, and call for adjustments at theplant if necessary.

■ Expect concrete to set more rapidly and havea shorter finishing time.

■ Ensure that slabs have a ‘minimum’ front towhich fresh batches of concrete are added.

■ Perform all operations rapidly, but don’tfinish slabs prematurely, eg while bleedwater is still on the surface.

■ Evaporation Control Protect the surface ofconcrete slabs at all stages against excessiveevaporation and premature drying out by usingan evaporative retarder such as aliphaticalcohol. Aliphatic alcohols are applied to thefreshly placed and screeded concrete surface,where they form a chemical film which reducesthe rate at which water evaporates from thesurface of the concrete. By controlling thepremature drying out of the surface layer ofconcrete, the tendency for the concrete to shrinkand for plastic shrinkage cracking to occur isreduced or eliminated. The benefits of using aliphatic alcohols on theconcrete surface can be manifested in manyways as listed below. These benefits are usuallyrealised with no effect on the cement hydrationand/or setting time of the concrete.■ Improves surface workability and thereby

increases project and labour productivity.■ Improves surface finishing, and in turn

durability, abrasion resistance andresistance to the ingress of water-bornecontaminants.

■ Assists in eliminating the damaging practiceof ‘wet wiping’ or spraying water onto the drypatches during trowelling of the concrete,and thus helps to prevent surface dustingproblems.

■ Significantly reduces surface crusting.Surface crusting can cause problems withcracking, particularly with stamped concretefinishes.

■ Reduces the risk of premature surfacefinishing – by allowing the concrete toproperly set. A dry surface layer may appearto indicate that the concrete has set, but canlead to a ‘spongy concrete’ effect, as theconcrete below is still plastic. This may resultin flaking and an uneven surface finish.

■ Virtually eliminates differential surfacedrying.

Aliphatic alcohols have an average coverage of60 to 80 m2 per litre. Because the dilution ratevaries depending on the specific product, theapplication rate should always be in accordancewith the manufacturer’s specifications.In adverse or severe weather conditions, or insituations where the surface is worked (or thecoverage is otherwise broken), the aliphaticalcohol may need to be re-applied to maintain aneffective cover (ie after bullfloating, trowellingand other surface floating). In extreme conditionsthere may be a requirement for 5 to 6 or moreapplications to prevent the rapid drying of thesurface while the concrete sets and bleed waterdissipates.Most aliphatic alcohols contain a ‘fugitive dye’ toassist the monitoring of coverage. The dye has noeffect on the strength or colour of the concrete.

■ Curing and Protection It is essential that allsurfaces be kept continuously moist by curingthe concrete, since drying, even intermittently,can produce drying shrinkage and/or crazingtype cracking on the concrete surface.■ Curing should commence immediately after

the slab has been finished, and is particularlyimportant during the first day afterplacement, and in hot or windy conditions.

■ Curing methods include ponding with water,use of wet hessian or cotton mats,continuous spray mist, covering with plasticsheeting or sprayed on curing compounds.

■ When forms are removed, curing should beprovided to the newly exposed surfaces.

■ Some means of water curing is advantageousin hot weather as the water also assists incooling the concrete while it hardens andgains strength. Care is required, however, asthe surface needs to be sufficiently hardbefore water curing can be undertakenwithout risk of surface damage. This mayleave the surface exposed to drying andpossible cracking between completion of thefinishing and commencement of the curing.

■ Note that aliphatic alcohols are NOT curingcompounds and should not be specified orused as a substitute for them. Aliphaticalcohols are used between the initialscreeding and final finishing operations, andadequate curing of the concrete must still beprovided once final finishing has beencompleted. While aliphatic alcohols arecompatible with most curing compounds,this should be checked if products arepurchased from different manufacturers.

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REFERENCES1 AS 1379 Manufacture and supply of concrete

Standards Australia 19972 ACI Committee 305, Hot Weather Concreting

ACI Manual of Concrete Practice Part 2:Construction practices and inspection,pavements American Concrete Institute,Farmington Hills, USA, 1999.

This data sheet replaces:Technical Bulletin 95/2 Hot Weather Concretingpreviously published by the Australian Pre-MixedConcrete Association.

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NOV2004

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