How do I know if it's safe to pour concrete today?

Four numbers decide it: air temperature, relative humidity, wind speed about 18 inches above the slab, and concrete temperature. Run them through the ACI 305R evaporation formula — anything over 0.15 lb/ft²/hr means you are flirting with plastic shrinkage cracking. PourDay does the math live using weather data for your exact job site and gives a GO, CAUTION, or WARNING.

When should you NOT pour concrete?

Three situations to walk away from: (1) Air temperature below 40°F and falling — hydration stops and the slab will not gain strength. (2) Air temperature above 90°F with low humidity and wind — evaporation runs away and the surface skins over before finishing. (3) Heavy rain in the forecast within 4-8 hours of placement, which washes out finish and air content.

How long does concrete take to cure?

Initial set is 4-8 hours. Concrete reaches roughly 65-75% of design strength at 7 days, and 100% of design strength at 28 days — the standard benchmark for cylinder breaks. Hydration continues for months but 28 days is the structural number. These figures assume adequate moisture during curing; high evaporation during placement can permanently compromise strength.

Why does concrete crack?

Most premature cracking traces back to pour day: high evaporation rate causing plastic shrinkage cracking, excessive heat accelerating the set, or restraint cracks from improper jointing. The first two are the ones contractors can prevent by reading weather correctly before placement.

What temperature is too hot to pour concrete?

There is no single temperature — it is the combination of air temp, humidity, wind, and concrete temperature. 95°F with 60% humidity and calm wind can be fine. 85°F with 20% humidity and 15 mph wind is dangerous. ACI 305R uses the evaporation rate calculation to give the real answer.

What is a safe evaporation rate for concrete?

ACI 305R recommends precautions when evaporation rate exceeds 0.15 lb/ft²/hr. Above this threshold, surface moisture evaporates faster than bleed water can replace it, causing plastic shrinkage cracking — especially with blended cements like Type 1L which have less bleed water.

Yes, PourDay is completely free for contractors. No subscription, no paywall, no premium tier.

Can I track multiple job sites?

Yes, PourDay supports up to 10 saved locations. Each location gets its own real-time evaporation rate, 16-day forecast, and pour condition scoring.

What do GO, CAUTION, and WARNING mean?

GO means the evaporation rate is below 0.10 lb/ft²/hr and conditions are favorable. CAUTION (0.10–0.20) means take precautions like windbreaks, fogging, or evaporation retarders. WARNING (above 0.20) means the rate exceeds safe limits and you should mitigate aggressively or postpone the pour.

How does PourDay know my pour conditions?

PourDay pulls live, hyper-local weather data for your exact job site — air temperature, humidity, and wind — and runs it through the ACI 305R evaporation rate formula automatically, updating throughout the day.

Yes, PourDay is completely free for contractors. No subscription, no paywall, no premium tier.

How It Works

See your pour conditions. Know before you break ground.

PourDay reads the weather at your exact job site and turns it into one clear answer: GO, CAUTION, or WARNING. Real-time, built on the ACI 305R evaporation rate formula — so the call gets made before the truck leaves the yard, not after the slab cracks.

The Process

Four steps. Zero guesswork.

Set your job site

Enter a zip code or drop a pin. PourDay pulls hyper-local weather for that exact spot — anywhere in the US.

It reads the four variables

Air temperature, relative humidity, wind speed at slab level, and concrete temperature — the inputs that actually decide a pour.

It runs the ACI 305R formula

PourDay computes your evaporation rate in lb/ft²/hr automatically and continuously. No nomograph, no spreadsheet, no math.

You get GO, CAUTION, or WARNING

One clear decision. Pour, take precautions, or push the pour — with the number behind it if you want to see it.

The Scoring

What the colors mean.

GO

Evaporation rate below 0.10 lb/ft²/hr. Conditions are favorable for placing concrete — standard curing practices apply.

CAUTION

0.10–0.20 lb/ft²/hr. Elevated risk. Use windbreaks, fogging, evaporation retarders, or shift your pour to a cooler part of the day.

WARNING

Above 0.20 lb/ft²/hr. You're past the safe limit — mitigate aggressively or push the pour. This is the line where slabs crack.

Want to see the math behind it? Try the free evaporation rate calculator or read ACI 305R explained.

More Than Today

Plan ahead, then document it.

Pour conditions aren't just a today problem. PourDay also gives you a 16-day forecast for every saved job site so you can schedule around high-evaporation days, and a pour log to record conditions, mix design, and outcome — the paper trail that protects your work when a crack shows up six months later.

Know before you break ground.

Real-time pour conditions, the 16-day forecast, and pour logging — free for contractors on iOS and Android.

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