Evaporation Rate
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kg/m²/hr (ACI 305R formula)
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The complete professional guide to understanding, predicting, and preventing plastic shrinkage cracks in concrete
Master plastic shrinkage cracking prevention with the ACI 305 evaporation rate checker, step-by-step on-site protocols, windbreak and misting strategies, evaporation retarder application guides, and complete reference tables for 2026.
Why evaporation rate is the single most critical variable to control during fresh concrete placement
Plastic shrinkage cracks occur while concrete is still in its plastic state — before final set — when the rate of surface evaporation exceeds the rate at which bleed water rises to replace it. As the surface dries faster than the interior, tensile stresses develop in the fresh concrete skin and, because the material has virtually no tensile strength at this stage, cracks form. These cracks are typically diagonal, parallel, or random in pattern and can penetrate the full slab depth.
According to ACI 305R — Hot Weather Concreting, when the evaporation rate from the concrete surface reaches or exceeds 1.0 kg/m²/hr (0.20 lb/ft²/hr), precautionary measures are mandatory. At rates above 1.5 kg/m²/hr, extreme precautions including erecting windbreaks, applying evaporation retarder, and continuous fogging are all required simultaneously. The evaporation rate is driven by four variables: concrete temperature, air temperature, relative humidity, and wind speed.
Effective plastic shrinkage cracking prevention in 2026 relies on a three-tier approach: pre-placement planning (scheduling pours to avoid high-evaporation periods, pre-wetting sub-bases), active during-placement controls (windbreaks, fogging mist systems, evaporation retarder sprays), and immediate post-placement curing (wet hessian, plastic sheeting, curing compounds applied within minutes of final trowel). Missing any tier significantly increases crack risk even when the others are in place.
Enter site conditions to calculate evaporation rate and plastic shrinkage cracking risk level — based on ACI 305R 2026
Plastic shrinkage cracks are formed in fresh concrete — typically within the first 1 to 8 hours after placement — before the concrete has developed any meaningful tensile strength. The mechanism is straightforward: bleed water normally migrates upward through the fresh mix and keeps the surface moist. When surface evaporation outpaces this bleed water supply, the surface concrete begins to dry and contract while the interior remains wet and restrained. The differential shrinkage creates tensile stresses that the paste-rich surface layer cannot resist.
Unlike drying shrinkage cracks (which form days or weeks after placement as the hardened concrete loses moisture), plastic shrinkage cracks can appear within minutes of the concrete surface losing its bleed water sheen. They often go unnoticed during the busy finishing phase and are only discovered after the concrete has hardened. By then, the cracks — which may be 1–3 mm wide and extend 50–150 mm deep — cannot be reversed. Effective plastic shrinkage cracking prevention therefore requires proactive monitoring of site conditions before and during the pour, not reactive intervention after cracks appear.
Plastic shrinkage cracking prevention must begin before the first truck arrives on site. Once cracks appear in fresh concrete, they cannot be closed by re-trowelling — compressing a crack forces aggregate particles apart and leaves a permanent weakened plane. The only effective strategy is prevention. Check evaporation rate conditions using the tool above every 30 minutes during the pour on high-risk days.
Risk zones based on ACI 305R evaporation rate thresholds — metric values (kg/m²/hr) — 2026 reference
Plastic shrinkage cracking is ultimately caused by a single physical event — surface moisture loss exceeding bleed water supply — but five site-specific variables determine whether this event occurs and how severe it becomes. Understanding each variable is essential for effective plastic shrinkage cracking prevention on any project.
Wind is the most powerful accelerator of evaporation. Even a moderate breeze of 15–20 km/h can more than double the evaporation rate compared to calm conditions. On exposed sites — road slabs, airport aprons, open warehouse floors — wind speed should be measured at slab level, not at standard meteorological height, as ground-level wind is often significantly higher. Windbreaks are typically the single most effective plastic shrinkage cracking prevention measure on exposed sites.
Higher concrete temperatures increase the vapour pressure at the slab surface, directly raising evaporation rate. A concrete temperature of 35°C produces dramatically more surface evaporation than a 20°C mix under identical ambient conditions. Reducing concrete temperature by 5–8°C through ice substitution for mix water, shaded aggregate stockpiles, and chilled water can reduce evaporation rate by 20–35%, making it one of the most cost-effective plastic shrinkage cracking prevention strategies for hot weather concreting.
High air temperature reduces the vapour pressure differential between the concrete surface and the ambient air, allowing more evaporation to occur. When air temperature exceeds concrete temperature — a common scenario in hot weather when the sun heats the air faster than it heats the concrete mix — evaporation rates can reach extreme levels within minutes of placement. Scheduling pours in the early morning or late afternoon hours is one of the simplest plastic shrinkage cracking prevention strategies available.
Dry air — below 40–50% relative humidity — significantly increases the capacity of the atmosphere to absorb moisture from the concrete surface. In arid climates, desert regions, and air-conditioned interior environments, relative humidity below 20–30% can create critical evaporation rates even when wind and temperature are moderate. Humidity cannot easily be controlled on outdoor sites, making it a primary driver for scheduling decisions when conducting plastic shrinkage cracking prevention planning.
Direct sunlight heats the concrete surface, significantly increasing the surface temperature above air temperature and dramatically raising local evaporation rate. A concrete slab in direct summer sun can reach surface temperatures 10–15°C above air temperature within 30 minutes of placement. Shading freshly placed concrete using temporary fabric shade structures or scheduling pours to avoid peak solar hours (10:00–15:00) is an underutilised plastic shrinkage cracking prevention strategy.
Modern concrete mixes — particularly those with silica fume, fly ash, low water-to-cement ratios, or high-range water reducers — produce very little bleed water. While these admixtures improve long-term concrete quality, they also eliminate the natural "buffer" of surface bleed water that normally compensates for moderate evaporation. Low-bleed mixes require more aggressive and faster-acting plastic shrinkage cracking prevention measures than standard mixes, as the surface can lose its moist sheen within minutes of placement.
The ACI 305R evaporation rate formula, originally developed by Menzel (1954) and refined in subsequent ACI committee updates, provides a reliable method for estimating evaporation rate from fresh concrete surfaces based on four measurable site parameters. This formula is the basis of the risk checker tool above and forms the technical foundation of all professional plastic shrinkage cracking prevention planning.
The ACI 305R formula assumes no direct solar radiation input and standard atmospheric pressure. On bright sunny days, actual evaporation rates at the slab surface may be 20–40% higher than the formula predicts because solar radiation heats the surface concrete beyond the measured air temperature. When direct sun is present, add a conservative safety margin and treat the calculated evaporation rate as a minimum, not a maximum estimate.
The following protocol integrates all three prevention tiers — pre-placement, during placement, and post-placement — into a structured on-site workflow. This approach aligns with the recommendations of ACI 305R, AS 3600, and the Concrete Institute of Australia's current 2026 practice notes for plastic shrinkage cracking prevention.
Obtain the next-day weather forecast including air temperature, relative humidity, wind speed, and solar conditions. Input the values into the evaporation rate checker above using your expected concrete placement temperature. If the calculated evaporation rate approaches or exceeds 0.75 kg/m²/hr, initiate contingency planning for windbreaks, fogging equipment, and evaporation retarder procurement.
Request chilled or iced mix water from the batching plant on hot days. Pre-wet and shade aggregate stockpiles 24 hours before the pour. Specify a target concrete delivery temperature of ≤ 28°C (ACI 305R recommendation). Each 5°C reduction in concrete temperature reduces evaporation rate by approximately 15–20%. If concrete temperature exceeds 32°C on arrival, consider rejecting the load or requesting a mix temperature correction.
Thoroughly wet the sub-base, existing concrete base, and formwork immediately before placing concrete. A dry sub-base will absorb bleed water from the fresh concrete, further reducing the surface moisture buffer and increasing plastic shrinkage cracking risk. Do not allow standing water — the surface should be damp but no free water should be visible at the time of placement.
Position shade cloth windbreaks or solid hoarding panels upwind of the pour area before the first truck arrives. Windbreaks should extend at least 1.5 m above the slab surface and be positioned so they do not create turbulence over the concrete. A 50% reduction in wind speed reduces the evaporation rate by approximately 30–40%. On large pours, phased windbreak placement ahead of the advancing concrete face is often necessary.
Spray an aliphatic alcohol-based evaporation retarder onto the concrete surface as soon as possible after screeding and before any finishing operations begin. Apply in a fine, uniform mist — do not puddle. Reapply after each finishing pass if the surface sheen disappears before the next operation. Evaporation retarders do not prevent evaporation entirely; they slow it by 30–70% and must be used alongside other prevention measures, not as a standalone solution.
For high-risk conditions (evaporation rate ≥ 1.0 kg/m²/hr), set up fogging or fine mist irrigation systems upwind of the pour to increase ambient humidity immediately above the slab surface. The mist droplets should be fine enough to evaporate before reaching the concrete surface — water droplets landing directly on fresh concrete will mar the surface finish and can cause localised cracking. Maintain misting throughout the entire finishing period.
As soon as the concrete can no longer be marked by foot traffic, apply the specified curing system — either a liquid membrane-forming curing compound (ASTM C309 Type 1-D) at the manufacturer's specified application rate, or wet hessian covered by polyethylene sheeting. Do not delay — every minute between final trowel and curing application is additional uncontrolled evaporation time. A 30-minute delay in curing on a high-evaporation day can be enough to initiate plastic shrinkage cracking.
Multiple plastic shrinkage cracking prevention measures are available to the site team. The table below compares the most widely used methods by effectiveness, cost, ease of implementation, and applicability across different project types and scales. Most high-risk pours require a combination of at least two or three measures applied simultaneously.
| Prevention Method | Evaporation Reduction | Cost Level | Implementation Speed | Best For | Limitations |
|---|---|---|---|---|---|
| Windbreaks (shade cloth / hoarding) | 30–50% | Low–Medium | Setup before pour | Exposed slabs, open sites | Not effective against all wind directions |
| Evaporation Retarder Spray | 30–70% | Low | Immediate — spray after strike-off | All slab types, especially low-bleed mixes | Must be reapplied; cannot substitute curing |
| Fogging / Misting Systems | 20–40% | Medium | Set up before pour | Large pours, industrial floors, hot weather | Requires equipment; droplets must not land on slab |
| Reduce Concrete Temperature | 15–30% | Low–Medium | Batching plant — plan 24 hrs ahead | All projects in hot weather | Limited to ~5–8°C reduction practically |
| Shade Structures (solar exclusion) | 20–40% | Medium–High | Must be in place before pour | Interior slabs, covered structures | High setup cost; impractical for large areas |
| Pour Timing (early morning / night) | 30–60% | Nil | Planning stage only | All projects — most cost-effective measure | May not be feasible due to programme constraints |
| Immediate Wet Curing (hessian + plastic) | Eliminates post-trowel evaporation | Low | After final trowel — immediately | All slab types, final prevention layer | Cannot be applied during finishing operations |
| Polypropylene Fibres in Mix | Does not reduce evaporation | Low | Specified at design stage | High-risk environments — reinforces plastic zone | Reduces but does not eliminate crack risk |
The addition of monofilament polypropylene (PP) fibres to the concrete mix is one of the most widely adopted passive measures for plastic shrinkage cracking prevention. At dosage rates of 0.6–1.0 kg/m³, PP fibres create a three-dimensional reinforcing matrix throughout the fresh concrete that bridges micro-cracks as they initiate, redistributing stress and limiting crack widths. Fibres do not prevent evaporation or reduce the driving force of plastic shrinkage — they simply increase the tensile capacity of the paste during the critical plastic window.
Importantly, PP fibres must be monofilament type (single-strand) for effective plastic shrinkage cracking prevention — fibrillated fibres are less effective in this application. Fibres should be uniformly dispersed during mixing, which typically requires an additional 1–2 minutes of mixing time. Surface balling of fibres during trowelling is a common complaint and can be managed by using lower dosage rates (0.6 kg/m³) and ensuring thorough mixing before placement.
The table below provides pre-calculated evaporation rates using the ACI 305R formula for a range of typical site conditions encountered in 2026. Use this as a quick field reference to assess plastic shrinkage cracking risk before conducting a full calculation with the tool above. All values assume a concrete placement temperature of 25°C and calm wind (10 km/h) unless stated.
| Scenario | Air Temp (°C) | RH (%) | Wind (km/h) | Concrete Temp (°C) | Evaporation Rate | Risk Level |
|---|---|---|---|---|---|---|
| Mild overcast morning | 18 | 70 | 8 | 22 | 0.20 kg/m²/hr | ✅ Low |
| Warm spring day | 24 | 55 | 12 | 25 | 0.45 kg/m²/hr | ✅ Low |
| Warm day, moderate wind | 28 | 45 | 20 | 28 | 0.82 kg/m²/hr | ⚠️ Moderate |
| Hot day, low humidity | 35 | 30 | 15 | 30 | 1.18 kg/m²/hr | ❌ High |
| Hot, dry, windy — classic risk day | 38 | 20 | 30 | 32 | 1.95 kg/m²/hr | 🚨 Extreme |
| Hot but high humidity (tropical) | 34 | 80 | 10 | 30 | 0.38 kg/m²/hr | ✅ Low |
| Interior air-conditioned slab | 22 | 25 | 5 | 24 | 0.62 kg/m²/hr | ⚠️ Moderate |
| Desert climate afternoon | 42 | 15 | 25 | 35 | 2.60 kg/m²/hr | 🚨 Extreme |
Despite best efforts, plastic shrinkage cracks can still form on high-risk days. When cracks are observed in fresh concrete, the site team must act immediately — the response window is narrow and the available remedies depend on how recently the cracks formed relative to the initial set of the concrete.
The primary ACI guide for hot weather concreting, covering evaporation rate calculation, plastic shrinkage cracking prevention measures, mix design modifications, and temperature management for fresh concrete in hot and dry conditions.
View ACI 305R →Comprehensive guide to curing methods for concrete, including evaporation retarders, wet curing, and membrane-forming curing compounds. Essential companion document for any plastic shrinkage cracking prevention programme.
View ACI 308R →Standard specification for liquid membrane-forming compounds used for curing concrete. Defines Type 1, 1-D, 2, and 2-W classifications used in plastic shrinkage cracking prevention and post-placement curing programmes.
View ASTM C309 →