ACI 305R is the American Concrete Institute's guide for hot weather concreting. It covers everything from batching and mixing to placing, finishing, and curing concrete in conditions that accelerate moisture loss and hydration — conditions that are common across most of the US for at least half the year.
The standard defines “hot weather” broadly: any combination of high air temperature, low relative humidity, wind, and solar radiation that tends to impair the quality of freshly mixed or hardened concrete. It is not just about temperature. A 75°F day with low humidity and high wind can be more dangerous than a 95°F day with high humidity and calm air.
The centerpiece of ACI 305R is the evaporation rate estimation. The formula takes four field-measurable inputs — air temperature, concrete temperature, relative humidity, and wind speed — and produces a single number in lb/ft²/hr that tells you how fast moisture is leaving the concrete surface.
Historically, contractors used the ACI nomograph — a graphical chart where you plot each variable and follow lines to find the resulting evaporation rate. The nomograph is accurate but slow and impractical in the field. Most contractors have never used one.
For a detailed breakdown of each variable and how they interact, see our complete evaporation rate guide. The key insight of the formula is that it combines variables that interact in non-obvious ways. You cannot assess evaporation risk by looking at any single weather measurement. Temperature, humidity, wind, and concrete temperature all contribute, and their combined effect is what determines whether your pour is safe.
The standard recommends precautions when the calculated evaporation rate exceeds 0.15 lb/ft²/hr. This threshold is referenced in most commercial concrete specifications and is the benchmark that inspectors, engineers, and quality control teams use to evaluate pour conditions. Above it, the risk of plastic shrinkage cracking increases significantly.
Precautions include: scheduling pours to avoid peak evaporation conditions, using evaporation retarders on the fresh concrete surface, erecting windbreaks around the pour area, fogging the air above the slab to raise local humidity, requesting lower concrete delivery temperatures from the ready-mix supplier, and beginning curing immediately after finishing.
The standard does not mandate canceling a pour above 0.15 — it requires precautions. But many contractors and specifications set their own thresholds, and some treat anything above 0.20 as a no-pour condition without aggressive mitigation in place.
On the job site, ACI 305R guidance translates into practical decisions that happen before and during every pour:
Before the pour: Check evaporation rate for your job site. If it exceeds your threshold, decide whether to reschedule, order mitigation materials, or adjust the mix (request a retarder, lower concrete temperature). Stage windbreaks, evaporation retarder spray, and curing compound at the pour location.
During the pour: Monitor conditions as they change throughout the day. A pour that starts at 6am in safe conditions can cross the 0.15 threshold by 10am as temperature rises and humidity drops. Apply evaporation retarder after each finishing pass. Start curing immediately after final finish.
After the pour: Document conditions. Your pour log should show the evaporation rate, weather conditions, and any mitigation measures used. This protects you if questions arise about surface quality later.
The shift from traditional Type I/II Portland cement to Type 1L (Portland-limestone cement) has made ACI 305R more critical than ever. Type 1L is now the default cement in much of the US, driven by sustainability targets — it produces 10% less CO₂ per ton than traditional Portland cement.
The tradeoff: Type 1L produces less bleed water. Bleed water is the thin film of moisture that rises to the concrete surface after placement, and it is your natural protection against evaporation. With less bleed water, the surface dries out faster, and the 0.15 lb/ft²/hr threshold hits harder. Contractors who never worried about evaporation rate with Type I/II are now seeing plastic shrinkage cracks with Type 1L under the same conditions.
This is the single biggest reason evaporation rate monitoring has moved from “nice to have” to “essential.” The cement changed, the risk profile changed, and the old gut-feel approach no longer works.
ACI 305R is a technical document written for engineers, but its core message is simple: know your evaporation rate before you pour. Try the formula yourself with our free evaporation rate calculator, or let PourDay automate it using live weather data for your exact job site. Open the app, confirm your location, and you have your evaporation rate and GO/CAUTION/NO GO status in seconds — no nomograph, no formula, no guesswork.
Apply ACI 305R on every pour without reading the standard. PourDay does the math — free on iOS and Android.
Deep dive into the evaporation rate formula — what each variable does and how they interact on the job.
Read articleThe consequence of ignoring ACI 305R — what causes cracks, the 0.15 rule, and field prevention methods.
Read articleWhy conditions at placement matter more than your curing plan — and how to get both right.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.