Arizona Evaporation Rate Pour Window Reference

Why this matters

In Phoenix, Tucson, and the Sonoran low desert, the seasonal evaporation rate routinely exceeds the ACI 305 plastic-shrinkage cracking threshold of 0.2 lb/sq ft/hr for 6-8 months of the year. A crew that pours at 11 a.m. on a June day in Phoenix without an evaporation-rate calculation has placed a slab that will craze within 4 hours. ACI 305.1-14 is the national standard; Arizona's specific climate combination of high temperature, low humidity, and wind drives the calculation in ways that crews in the Midwest do not see. This article covers the local pour-window decisions: when to pour, when to refuse to pour, and what the published threshold means at typical Arizona summer site conditions.

The ACI 305 evaporation chart

ACI 305.1-14 (Figure 2.1.5) plots evaporation rate as a function of four variables:

1. Concrete temperature at placement.
2. Air temperature.
3. Relative humidity.
4. Wind speed.

The chart returns a rate in lb/sq ft/hr. Above 0.2 lb/sq ft/hr, plastic shrinkage cracking is expected unless protective measures are taken. Above 0.1 lb/sq ft/hr, protective measures are recommended.

Typical Phoenix June afternoon: 105 F air, 15 percent RH, 10 mph wind, concrete temperature 85 F. Reading the ACI 305 chart returns an evaporation rate of approximately 0.4-0.5 lb/sq ft/hr, twice the cracking threshold. Plastic shrinkage cracking is essentially guaranteed without aggressive evaporation control.

Typical Phoenix April morning: 75 F air, 30 percent RH, 5 mph wind, concrete temperature 70 F. Same chart returns approximately 0.10 lb/sq ft/hr. Below the action threshold; protective measures still good practice but not survival-critical.

Decision 1: Time of day

Arizona crews schedule pours to land outside the high-evaporation window. The practical rules:

• May through September: place by 8 a.m., finish before 11 a.m. Afternoon placements are not viable for unprotected slabs.
• April and October: morning placements preferred; mid-day acceptable with protective measures.
• November through March: full-day window typically available with minimal protective measures.

Night pours on commercial work are common in summer. Start at 7 p.m., finish at midnight, cure overnight. Slabs poured at 90 F ambient at 8 p.m. with 25 percent RH and falling temperature reach the action threshold but do not exceed the cracking threshold.

Decision 2: Concrete temperature management

ACI 305 sets a recommended maximum concrete temperature of 95 F at placement; some specifications drop this to 90 F. In Arizona summer, achieving 95 F at placement requires active management.

Ice substitution: replace up to 75 percent of the batch water with ice at the plant. Cool water cooler than 50 F is the more common practice. Cooling cement (cement straight from the silo can be 150 F+) by waiting for night batches.

Aggregate cooling: spray aggregate piles with water in the early morning before batching. Evaporative cooling drops aggregate temperature 10-20 F.

Liquid nitrogen injection: high-end commercial pours where concrete temperature is contractually capped at 70 F or lower.

For residential work in Arizona summer, the practical limit is morning batching with cool water. The plant typically holds water below 60 F in summer; cement is what it is; aggregate is shaded if possible. Achieving 90 F concrete on a 105 F afternoon is rarely possible without contractor-side intervention; the schedule moves to morning instead.

Decision 3: Wind protection

The ACI 305 chart shows wind speed has the largest single-variable impact on evaporation. Going from 5 mph to 15 mph at constant temperature and humidity can double the evaporation rate.

Wind breaks (snow fence, 6 ft plywood barriers, structural building walls) at the upwind side of the pour drop site-level wind from prevailing 10-15 mph to 2-5 mph. Effective on slab pours up to 1,000 sq ft. Above that area, wind break is impractical and the crew must rely on other measures.

In Phoenix, the prevailing daytime wind is from the west-southwest at 5-15 mph in summer. A slab pour on an east-facing lot side may have natural building shielding; a west-facing pour has none.

Decision 4: Evaporation retarders

Spray-on evaporation retarders (Sika Antisol E, MasterKure ER 50, or comparable) form a thin polymeric film on the surface that slows evaporation. Applied immediately after bull floating and before bleed-water rise is complete. The film evaporates as bleed water rises, leaving a clean surface for finishing.

Critical use rules:

1. Apply as a fine mist, not a pool. Pooling creates a non-uniform film and may interfere with finishing.
2. Reapply after each finishing pass that breaks the film.
3. Not a curing compound. Curing compound goes on after finishing is complete. Evaporation retarder is between operations during placement.

For Arizona summer pours, evaporation retarder is standard practice on every flatwork pour. Spray after bull float, finish, spray again before troweling.

Decision 5: Fog spray

A fine fog of water suspended in the air above the slab raises localized humidity from 15 percent ambient to 80-100 percent. Drops the evaporation rate from the chart value to near zero.

Setup: a misting wand off a garden hose (Dramm Touch-N-Flow or comparable) at 50 psi water pressure, held 3-6 ft above the slab, creates an effective fog. Two wands on a 1,000 sq ft pour with the crew rotating one finisher to fog while the others place is typical practice.

Critical rule: fog the air above the slab, not the slab surface. Wetting the surface drives more bleed water than was lost; the slab gets weaker from added water in the cream layer.

Decision 6: Cure regime

ACI 308.1 requires moist curing for 7 days for typical concrete to develop design strength. In Arizona, dry curing is the default the climate wants; the crew must override.

Standard Arizona curing options:

1. Curing compound (resin-based, ASTM C309 compliant) sprayed after finishing. Forms a film that retains moisture for 5-7 days.
2. Plastic sheeting (ASTM C171) laid over the slab and weighted at edges. Retains moisture as long as the sheet stays in contact with the surface.
3. Wet burlap kept continuously wet for 7 days. Highest-quality cure but requires staffing to wet the burlap multiple times per day in summer.

For residential driveways and patios in Arizona, curing compound is the typical practice; plastic sheeting is the backup for high-spec work. ACI 308 recommends plastic over curing compound for the longer protection window.

References

• ACI 305.1-14, Specification for Hot Weather Concreting, American Concrete Institute, Figure 2.1.5.
• ACI 305R-20, Guide to Hot Weather Concreting.
• ACI 308.1-11, Specification for Curing Concrete.
• ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete.
• ASTM C171, Standard Specification for Sheet Materials for Curing Concrete.
• 2018 Arizona International Building Code adoption per A.R.S. 41-2156 (state-level adoption of IBC/IRC with local amendments).