Hot-weather concreting is the set of mix, cooling and placing measures that keep fresh concrete inside a temperature limit, and in the US that limit is 35 °C (95 °F) as delivered under ACI SPEC-301-20 4.2.2.5(b). Heat, low humidity and wind speed up both moisture loss and cement hydration, so the mix sets sooner and loses water faster than the specification assumes.
Producers answer with two levers. The first is chemical: retarding and water-reducing retarding admixtures delay the set, and extended set-controlling admixtures stop hydration on long hauls. The second is thermal: chilled water, ice or liquid nitrogen lower the temperature of the mix before it leaves the plant. Concreting in hot weather is therefore as much a choice of concrete additives as a choice of schedule.
The limit depends on where the concrete is placed. Florida triggers hot-weather measures above 29.4 °C (85 °F) and rejects concrete above 37.8 °C (100 °F); India keeps placing temperature below 40 °C (104 °F). NRMCA CIP 12 credits chilled water with up to 6 °C (10 °F) of cooling and ice with up to 12 °C (20 °F). In the worked example below, chilling the mixing water to 5 °C (41 °F) lowers a mix by 5.1 °C (9.1 °F), and surface precautions start as evaporation approaches 1.0 kg/m²/h (0.2 lb/ft²/h).
Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).
The table below collects the attributes that define hot-weather concreting and the document behind each value.
| Attribute | Value |
|---|---|
| Definition source | ACI 305R-20 section 2.2 (sold as ACI PRC-305-20); NRMCA CIP 12 adds solar radiation |
| US limit | 35 °C (95 °F) maximum concrete temperature as delivered, ACI SPEC-301-20 4.2.2.5(b) |
| India | hot weather above 40 °C (104 °F); placing temperature below 40 °C, IS 7861 (Part 1):1975 |
| Florida | measures above 29.4 °C (85 °F); rejection above 37.8 °C (100 °F), FDOT Section 346-7.5 (FY 2024-25) |
| Admixture classes | ASTM C494/C494M-24 Types B, D and G; EN 934-2:2009+A1:2012 Table 8; IS 9103:1999 retarding type |
| Cooling options | chilled water up to 6 °C (10 °F); ice up to 12 °C (20 °F); liquid nitrogen for larger reductions (NRMCA CIP 12) |
| Evaporation threshold | 1.0 kg/m²/h (0.2 lb/ft²/h) (IS 7861 (Part 1) Appendix A; ACI 305R nomograph) |
| Related placements | self-compacting concrete loses flow in hot weather, long hauls and delays (NRMCA CIP 37) |
What Is Hot-Weather Concreting?#
ACI 305R-20 2.2 defines hot weather as "one or a combination of high ambient temperature, high concrete temperature, low relative humidity and high wind speed that tends to impair fresh or hardened concrete by accelerating moisture loss and cement hydration". NRMCA CIP 12 lists solar radiation as a fifth factor. The US definition is a set of conditions, not a number.
India and Florida set the threshold as a number. IS 7861 (Part 1):1975 clause 2.1 applies hot-weather practice when the atmospheric temperature, or the expected placing temperature, exceeds 40 °C (104 °F). FDOT Section 346-7.5 (REV 7-10-23, FY 2024-25) applies it when the concrete temperature at placing exceeds 29.4 °C (85 °F) and is 37.8 °C (100 °F) or less. Weather practice is one of the special concretes defined by how the mix is designed and controlled rather than by a single admixture.
What hot weather does to fresh and hardened concrete#
Hot weather raises the water a mix needs, speeds up cement hydration and widens the scatter of fresh and hardened properties. IS 7861 (Part 1):1975 Appendix A ties the extra water to rising concrete temperature. NRMCA TIP 18 states that a temperature variation above 5.5 °C (10 °F) changes fresh and hardened properties and raises the standard deviation of strength.
Faster hydration shortens working time, so set control comes first among the admixture decisions, and FHWA-HIF-18-017 adds a second risk in early stiffening with certain Class C fly ashes.
Which Admixtures and SCMs Control Concrete in Hot Weather?#
Set-controlling admixtures and lower-heat cementitious materials control concrete in hot weather: retarders delay the set, extended set-controlling admixtures stop hydration, accelerators speed finishing, and fly ash, slag cement and moderate-heat cements lower the heat. Each class below sits in the complete list of concrete admixtures with its ASTM, EN and IS designation.
The table below compares each component by its role in hot weather, its class or standard and the source that assigns that role.
| Component | Role in hot weather | Class or standard | Source |
|---|---|---|---|
| set-retarding admixtures | delay the set | ASTM C494/C494M-24 Type B; EN 934-2:2009+A1:2012 Table 8 | ASTM C494/C494M-24; EN 934-2 |
| water-reducing admixtures with retarding action | reduce water content at least 5 % and delay set | Type D | ACI PRC-212.3-16 Table 1.2 |
| superplasticizers (high-range water reducers), retarding | high-range water reduction with retardation | Type G | ASTM C494/C494M-24 |
| hydration-control admixtures | stop or severely retard hydration for long hauls | Type B or D | ACI PRC-212.3-16 Table 1.2 |
| workability-retaining admixtures | hold slump with no effect on initial slump, set time or strength gain | ACI PRC-212.3-16 class | ACI PRC-212.3-16 Table 1.2 |
| accelerating admixtures | expedite finishing; very rapid set in hot weather | Type C or E | NRMCA CIP 12; FHWA-HIF-18-017 |
| fly ash in concrete, slag cement (GGBS) | reduce hot-weather problems | supplementary cementitious materials | NRMCA CIP 12 |
| Type II moderate-heat or low-heat blended cement | reduce hot-weather problems; see heat of hydration | cement type | NRMCA CIP 12 |
| synthetic fibers, such as polypropylene microfibers | plastic shrinkage | fiber | NRMCA CIP 12 |
| lignosulfonates as water reducers | early stiffening with certain Class C fly ashes | Type D | FHWA-HIF-18-017 |
| effect to control: setting time of concrete | hot weather accelerates cement hydration | not a class | ACI 305R-20 2.2 |
NRMCA's guide to specifications (2PE004-21) names water-reducing, workability-retaining and retarding admixtures plus temperature reduction as the producer's tools for hot weather, and advises against delivery time limits stricter than ASTM C94/C94M.
Retarding and water-reducing retarding types#
Retarding admixtures are classed as ASTM C494/C494M-24 Type B (retarding), Type D (water-reducing and retarding) and Type G (water-reducing, high range, and retarding). EN 934-2:2009+A1:2012 Table 8 classes the set retarder by setting time: initial set at least control plus 90 min and final set at most control plus 360 min. IS 9103:1999 lists a retarding type alongside them. Its setting-time limits are not reproduced because the current Table 1 has not been read. Chemistry and dosage of each class are covered under set-retarding admixtures.
ACI PRC-212.3-16 (formerly ACI 212.3R-16, 76 pages) describes set-retarding admixtures in Table 1.2 as reducing water content at least 5 % and delaying set, and lists Type D in that row. EN 934-2 Table 10 covers the retarding plasticizer.
Stopping hydration for long hauls and delays#
Extended set-controlling admixtures stop or severely retard cement hydration, and ACI PRC-212.3-16 Table 1.2 assigns them to ASTM C494/C494M Type B or D. The same table lists their uses as wash water, returned concrete and long hauls, and their chemistry as carboxylic acids and phosphorus-containing organic acid salts. Where the haul is the problem but the set is not, ACI PRC-212.3-16 Table 1.2 lists workability-retaining admixtures, which keep slump with no effect on initial slump, set time or strength gain.
Accelerators, pozzolans and microfibers in hot weather#
NRMCA CIP 12 states that accelerators may be used in hot weather to expedite finishing and avoid plastic shrinkage cracking. FHWA-HIF-18-017 adds two cautions. Accelerators cause very rapid set in hot weather, and high doses of organic accelerators and calcium nitrate can extend set instead of shortening it.
The cementitious side of the mix carries more weight. NRMCA CIP 12 names Type II moderate-heat cement, low-heat blended cements, fly ash and slag cement as materials that reduce hot-weather problems, and synthetic fibers against plastic shrinkage. ACI PRC-305-20 gives fibers their own section, 4.10, and the fiber used for early surface cracking is covered under polypropylene microfibers.
Interactions that cause early stiffening#
FHWA documents two admixture interactions that stiffen hot concrete early. FHWA-HIF-18-017 reports that a lignosulfonate Type D water reducer combined with certain Class C fly ashes can cause early stiffening in hot weather; the listed remedies are to verify the material sources, change the admixture, cool the mix or delay the admixture addition. The FHWA MCTC water-reduction brief states that accelerating compounds in some water reducers can speed the aluminate phase and cause false or early set, worse in hot weather and with Class C fly ash.
How Is the Temperature of a Hot-Weather Mix Calculated and Lowered?#
The temperature of a fresh mix is calculated from the mass and temperature of each ingredient, with IS 7861 (Part 1):1975 clause 6.2 giving the equation in °C and kg. The equation has the same form as Appendix A of ACI 305R-20:
T = [0.22(Ta·Wa + Tc·Wc) + Tw·Ww + Ta·Wwa] / [0.22(Wa + Wc) + Ww + Wwa]
Ta, Tc and Tw are the temperatures of aggregate, cement and added water; Wa, Wc, Ww and Wwa are the masses of aggregate, cement, added water and free water on the aggregate. With ice, 79.6·Wi is subtracted in the numerator, Wi being the mass of ice.
Worked example (calculated, example inputs, not a mix design): 1,800 kg/m³ (3,034 lb/yd³) of dry aggregate at 30 °C (86 °F), 350 kg/m³ (590 lb/yd³) of cement at 60 °C (140 °F) and 160 kg/m³ (270 lb/yd³) of water at 25 °C (77 °F) give T = 20,500 / 633 = 32.4 °C (90.3 °F). With the water chilled to 5 °C (41 °F), the numerator falls to 17,300 and T = 27.3 °C (81.2 °F), 5.1 °C (9.1 °F) cooler. Adding the upper delivery rise of 3 °C (5 °F) from NRMCA TIP 18 gives 35.4 °C (95.7 °F) for the first mix and 30.3 °C (86.6 °F) for the chilled one.
The first mix arrives above the 35 °C (95 °F) limit of ACI SPEC-301-20; the chilled mix arrives inside it but above Florida's 29.4 °C (85 °F) trigger, so FDOT hot-weather measures still apply. Water carries a factor of 1 in the equation against 0.22 for aggregate and cement, so each kilogram of water moves the result about 4.5 times as much as a kilogram of solids.
The table below lists the example inputs and results.
| Ingredient or result | Mass kg/m³ (lb/yd³) | Temperature °C (°F) |
|---|---|---|
| Aggregate (dry) | 1,800 (3,034) | 30 (86) |
| Cement | 350 (590) | 60 (140) |
| Added water | 160 (270) | 25 (77) |
| Free water on aggregate | 0 (0) | n/a |
| Base mix, at the plant | n/a | 32.4 (90.3) |
| Chilled-water mix (water at 5 °C (41 °F)), at the plant | n/a | 27.3 (81.2) |
| Base mix after a 3 °C (5 °F) delivery rise | n/a | 35.4 (95.7) |
| Chilled-water mix after a 3 °C (5 °F) delivery rise | n/a | 30.3 (86.6) |
Calculated with the IS 7861 (Part 1) clause 6.2 equation; example inputs, not a mix design.
Chilled water, ice and liquid nitrogen#
Chilled water lowers concrete temperature by up to 6 °C (10 °F) and ice by up to 12 °C (20 °F), according to NRMCA CIP 12. Ice must be fully melted by the end of mixing (IS 7861 (Part 1) clause 4.3.2), and liquid nitrogen injected into the mixer gives larger reductions than ice (NRMCA CIP 12). The TxDOT study FHWA/TX-08/0-5111-1 (2007) found no significant effect on hydration or most properties, a slump that stays like that of hot concrete, and an extended set even when the nitrogen is applied just before initial set.
The table below compares the three cooling methods by reduction, condition and source.
| Method | Reduction | Condition or caution | Source |
|---|---|---|---|
| Chilled mixing water | up to 6 °C (10 °F) | water weighs most per kilogram in the mix equation | NRMCA CIP 12 |
| Ice | up to 12 °C (20 °F) | 80 kcal/kg (335 kJ/kg) absorbed on melting; fully melted by the end of mixing | NRMCA CIP 12; IS 7861 (Part 1) clause 4.3.2 |
| Ice for up to 80 % of the mixing water | up to about 11 °C (20 °F) | replaces part of the mixing water | PCA EB547 |
| Liquid nitrogen | larger than ice | injected into the mixer; setup cost; blade and drum precautions; can extend set | NRMCA CIP 12; FHWA/TX-08/0-5111-1 (2007) |
Water demand and consistence in the heat#
Hot concrete needs more water for the same slump, 2.25 to 2.5 % per 25 mm (1 in) of slump between 5 and 27 °C (41 and 81 °F) under IS 7861 (Part 1):1975 Appendix A. The same appendix gives 4.5 % at 49 °C (120 °F), water that raises the water/cement ratio unless an admixture replaces it.
Water reducers hold the slump without that water, and NRMCA CIP 12 sets the site rule that the design mixing water is never exceeded. How slump falls with time and temperature is covered under concrete workability.
What Temperature Limits Apply to Concrete in Hot Weather?#
The limit depends on the governing document: ACI SPEC-301-20 4.2.2.5(b) caps concrete at 35 °C (95 °F) as delivered, Florida rejects concrete above 37.8 °C (100 °F) and India keeps placing temperature below 40 °C (104 °F). ACI SPEC-301-20 permits a higher temperature where it has been used successfully in the local region or is shown by preconstruction testing, and the limit does not apply to mass concrete. ACI SPEC-305.1-14(20) sets 35 °C (95 °F) for fresh concrete at placement, higher if the architect/engineer accepts it after preconstruction testing. NRMCA TIP 18 states that ASTM C94/C94M sets no hot-weather limit on concrete as delivered.
In the UK, the BRMCA Ready Mixed Concrete Practical Guide (July 2020) gives a default maximum of 35 °C (95 °F) and 30 °C (86 °F) as the appropriate maximum for air-entrained concrete. EN 206-1:2026, current since CEN approval on 2026-01-19, deals with concrete temperature in clause 6.2.10; its values are not reproduced because the clause text has not been read.
The table below sets the limits side by side by jurisdiction, document and edition.
| Jurisdiction | Document (edition) | Rule | Value |
|---|---|---|---|
| US | ACI SPEC-301-20 4.2.2.5(b) | maximum as delivered; higher if proven locally or by preconstruction testing; not for mass concrete | 35 °C (95 °F) |
| US | ACI SPEC-305.1-14(20) | maximum fresh concrete at placement; higher if accepted by the architect/engineer after preconstruction testing | 35 °C (95 °F) |
| US | ASTM C94/C94M-26c | no hot-weather limit on concrete as delivered (NRMCA TIP 18) | none set |
| US-FL | FDOT Section 346-7.5 (REV 7-10-23, FY 2024-25) | hot-weather measures above 29.4 °C, else rejected; rejected above 37.8 °C regardless; pavement slab replacement mixes exempt from the 29.4 °C rule | 29.4 °C (85 °F); 37.8 °C (100 °F) |
| IN | IS 7861 (Part 1):1975 | placing temperature below; concrete reaches the forms not hotter than | 40 °C (104 °F) |
| IN | IS 7861 (Part 1):1975 | cement preferably not used above | about 77 °C (171 °F) |
| UK | BRMCA Practical Guide (July 2020) | default maximum; air-entrained concrete | 35 °C (95 °F); 30 °C (86 °F) |
| EU | EN 206-1:2026 clause 6.2.10 | clause titled "Concrete temperature" | not reproduced (clause text not read) |
Measuring the delivered temperature#
Concrete temperature is measured to ASTM C1064/C1064M-23, active since 2023-12-21, which allows up to 20 min for heat transfer where aggregate exceeds 75 mm (3 in). NRMCA TIP 18 states that concrete warms 0.5 to 3 °C (1 to 5 °F) during delivery and that the plant targets a temperature below the specified maximum. The C1064 procedure sits with the other fresh concrete tests.
Acceptance cylinders in hot weather#
Acceptance cylinders are initially cured at 16 to 27 °C (60 to 80 °F) per ASTM C31, and NRMCA CIP 12 notes that curing boxes with ice or refrigeration may be needed on hot sites.
How Is Concrete Placed and Protected in Hot Weather?#
Placing in hot weather is checked against the evaporation rate of the fresh surface and against the age of the concrete when water is added on site. The first governs surface protection, the second any water added to the load.
Evaporation rate and when to protect the surface#
Precautions start when the evaporation rate approaches 1.0 kg/m²/h (0.2 lb/ft²/h), the value in IS 7861 (Part 1):1975 Appendix A and in the ACI 305R nomograph. FHWA-HRT-05-038 treats 0.5 kg/m²/h (0.1 lb/ft²/h) as warranting caution and as little as 0.3 kg/m²/h (0.06 lb/ft²/h) as possibly critical for unprotected low-bleed paving concrete. ACI PRC-305-20 section 4.2 sets out how to calculate the estimated rate from air and concrete temperature, relative humidity and wind speed.
Evaporation-rate calculators, evaporation retarders, curing compounds and curing practice are construction-chemical topics not covered on this site. The mix side, from water content to fibers, is covered under plastic shrinkage cracking.
The table below compares the three evaporation thresholds and their sources.
| Evaporation rate kg/m²/h (lb/ft²/h) | Meaning | Source |
|---|---|---|
| 1.0 (0.2) | precautions when the rate approaches this value | IS 7861 (Part 1) Appendix A; ACI 305R nomograph |
| 0.5 (0.1) | warrants caution | FHWA-HRT-05-038 |
| 0.3 (0.06) | may be critical for unprotected low-bleed paving concrete | FHWA-HRT-05-038 |
Jobsite water and retempering limits#
NRMCA CIP 12 advises against adding water to concrete more than 1.5 hours old and against exceeding the mixing water of the design. The old 90-minute default of ASTM C94/C94M ended with C94/C94M-21, which requires the purchaser to state a time limit from the start of mixing to the completion of discharge (6.1.10). Where no limit is stated, the producer sets one and communicates it before delivery.
Which Standards and Guides Govern Hot-Weather Concreting?#
Hot-weather concreting is guided in the US by ACI 305R-20 and specified through ACI SPEC-305.1-14(20) and ACI SPEC-301-20, while India uses IS 7861 (Part 1):1975. The admixtures themselves are classed by ASTM C494/C494M-24, EN 934-2:2009+A1:2012 and IS 9103:1999. ACI 305R-20 was adopted and published in September 2020 and is sold as ACI PRC-305-20 (28 pages).
The table below lists each document with its edition, status and role.
| Region | Document (edition, status) | Role | Page |
|---|---|---|---|
| US | ACI 305R-20, sold as ACI PRC-305-20 (September 2020) | guide: practice, cooling, evaporation rate (4.2), fibers (4.10), temperature estimate (Appendix A) | this page |
| US | ACI SPEC-305.1-14(20) | specification: 35 °C (95 °F) at placement | this page |
| US | ACI SPEC-301-20 | specification: 35 °C (95 °F) as delivered, 4.2.2.5(b) | this page |
| US | ASTM C94/C94M-26c (active, 2026-08-12) | ready-mixed concrete; no hot-weather limit; purchaser time limit since -21 | admixtures in ready-mix concrete |
| US | ASTM C494/C494M-24 (active) | chemical admixtures, Types B, D, G | ASTM C494 chemical admixtures types |
| EU | EN 934-2:2009+A1:2012 | Table 8 set retarder; Table 10 retarding plasticizer | EN 934-2 admixture requirements |
| IN | IS 9103:1999 (active, reaffirmed 2018) | concrete admixtures, five types including retarding | IS 9103 concrete admixtures specification |
| Intl | ACI PRC-212.3-16 (formerly ACI 212.3R-16) | report: set-retarding, extended set-controlling and workability-retaining classes | ACI 212.3R report on chemical admixtures |
| IN | IS 7861 (Part 1):1975 (active, one amendment) | recommended practice for hot weather concreting | this page |
| US-FL | FDOT Section 346-7.5 (REV 7-10-23, FY 2024-25) | state specification: trigger and rejection temperatures | this page |
| US | ASTM C1064/C1064M-23 (active) | temperature of freshly mixed concrete | fresh concrete tests |
India: IS 7861 (Part 1) hot-weather rules#
IS 7861 (Part 1):1975, "Recommended practice for hot weather concreting", applies above 40 °C (104 °F) of atmospheric or expected placing temperature, and the BIS catalogue lists it as active with one amendment (checked 2026-10-26). The standard keeps placing temperature below 40 °C and requires concrete to reach the forms no hotter than 40 °C. Cement is preferably not used above about 77 °C (171 °F).
Admixtures for Indian work are specified to IS 9103:1999, active and reaffirmed in 2018, which covers concrete admixtures in five types: accelerating, retarding, water-reducing, air-entraining and superplasticizing. Its retarding type is the Indian counterpart of ASTM C494 Type B and EN 934-2 Table 8; its scope and amendments are covered under the IS 9103 concrete admixtures specification.
How to specify hot-weather concrete#
A hot-weather specification names five items so producer, contractor and laboratory work to the same limit. The five items are listed below.
- Maximum concrete temperature and the document that sets it (ACI SPEC-301-20, ACI SPEC-305.1-14(20), FDOT 346-7.5 or IS 7861 (Part 1)).
- Measuring point (as delivered or as placed) and method (ASTM C1064/C1064M-23).
- Admixture class (ASTM C494/C494M-24 Type B, D or G; EN 934-2:2009+A1:2012 Table 8; IS 9103:1999 retarding type).
- Cooling methods permitted (chilled water, ice, liquid nitrogen).
- Initial-curing range for acceptance cylinders, 16 to 27 °C (60 to 80 °F).
What Drives the Cost of Hot-Weather Concreting?#
The cost of hot-weather concreting is set by how far the mix must be cooled, the cooling method the plant runs, the set-controlling admixtures the haul needs and the risk of rejected loads. The four cost factors are listed below.
- Cooling method: chilled water and ice at the plant, or liquid nitrogen with its extra setup costs (NRMCA CIP 12).
- Admixture system: retarding, extended set-controlling or workability-retaining admixtures sized to the haul (ACI PRC-212.3-16 Table 1.2).
- Rejection risk: loads above 37.8 °C (100 °F) are rejected regardless of measures under FDOT Section 346-7.5.
- Testing provisions: curing boxes with ice or refrigeration for acceptance cylinders (NRMCA CIP 12).
How Does Hot-Weather Practice Relate to Mass Concrete and Winter Work?#
Hot-weather concreting ends where heat comes from inside the element rather than from the weather: ACI SPEC-301-20 excludes mass concrete from its 35 °C (95 °F) delivery limit. At the other end of the scale, ACI 306R-16 starts cold weather when the air temperature has fallen to, or is expected to fall below, 4 °C (40 °F) during the protection period, and accelerators replace retarders as the main admixture decision in cold-weather concreting.
NRMCA CIP 37 states that hot weather, long hauls and delays reduce the flow of self-compacting concrete.
Hot-weather concreting FAQs#
At what temperature can you no longer pour concrete?#
The cut-off depends on the governing document: 35 °C (95 °F) as delivered under ACI SPEC-301-20, rejection above 37.8 °C (100 °F) in Florida and placing below 40 °C (104 °F) in India. ACI SPEC-301-20 allows a higher value where local success or preconstruction testing supports it.
Can you pour concrete in really hot weather?#
Yes, concrete can be placed in very hot weather if it stays inside the governing temperature limit, which takes cooled ingredients and set-controlling admixtures. In the worked example on this page, chilled mixing water lowers the mix by 5.1 °C (9.1 °F).
What is ACI 305?#
ACI 305 refers to two ACI hot-weather documents: the guide ACI 305R-20 (ACI PRC-305-20, 28 pages, September 2020) and the specification ACI SPEC-305.1-14(20). The 35 °C (95 °F) maximum comes from the specification and from ACI SPEC-301-20, while the guide covers practice, cooling and the Appendix A temperature estimate.