Lesson 01 · Field guide
Hot-weather
concreting.
In Puerto Rico, “hot weather” is not just an air-temperature number. It is the combination of conditions that can accelerate moisture loss, hydration, setting, and construction risk.
01 · Definition
Hot weather is a combination of conditions.
ACI 305R-20 describes hot weather as high ambient temperature, high concrete temperature, low relative humidity, high wind speed, or a combination that tends to harm fresh or hardened concrete by accelerating moisture loss, hydration, or other detrimental effects.
Key idea: A windy, low-humidity day can create serious evaporation risk even when the air temperature alone does not look extreme.
02 · Why it matters
Heat changes both the material and the available working time.
Faster setting
Less time is available for transport, placement, consolidation, finishing, and corrective action.
Moisture loss
High temperature, low humidity, and wind can accelerate surface evaporation and raise plastic-shrinkage risk.
Workability loss
Slump can decrease more quickly, increasing pressure to add water or otherwise change the mixture at the site.
Strength & durability
A higher water demand, poor consolidation, delayed curing, or uncontrolled early-age temperature can compromise performance.
03 · Temperature
Know the specified limit—and how it will be verified.
ACI’s published technical FAQ states that ACI 301-20 and ACI 305.1-14 limit general hot-weather concrete to a maximum temperature of 95°F (35°C) at discharge. This statement is for general construction such as buildings, bridges, and pavements—not mass concrete.
A proposed maximum above 95°F requires successful field experience or preconstruction testing under ACI 305.1-14, with the request submitted to the architect/engineer before placement. Project specifications can impose a lower limit, so the contract documents control.
Confirm the project specification, measurement method, frequency, responsibility, and rejection/escalation path before placement.
04 · Plan before the pour
Control the risk as a system.
- Review the documents.Identify concrete-temperature limits, testing frequency, placement restrictions, curing requirements, and approval responsibilities.
- Study the forecast and exposure.Consider temperature, humidity, wind, sun, element geometry, access, haul time, and the placement rate—not only the daily high.
- Coordinate production and delivery.Discuss ingredient temperature, cooling measures, batching sequence, truck cycle, admixture strategy, and the process for site adjustments.
- Prepare the placement.Have labor, equipment, backups, test areas, fogging or evaporation-control measures, and curing materials ready before discharge.
- Protect immediately.Begin the approved curing and protection sequence as soon as finishing and the selected method permit. Prevent uncontrolled moisture loss.
05 · Field checklist
Before the first truck.
Do not use this checklist as a project specification. The architect/engineer’s requirements, approved submittals, and current contract documents govern the work.
06 · Worker heat safety
Protecting concrete never outranks protecting people.
Concrete hot-weather planning and worker heat-illness prevention are related but separate responsibilities. OSHA emphasizes water, rest, shade, acclimatization, training, emergency planning, schedule adjustments, and monitoring workers for signs of illness.
07 · Official references
Continue with the source documents.
This lesson is an original educational summary. ACI documents are copyrighted and should be obtained directly from ACI. Always verify the edition adopted by the project and check for errata.