Determine the risk of plastic shrinkage cracking based on ACI 305R guidelines.
Evaporation Rate
0.00
lb/ft² per hour
function calculateEvaporation() {
// 1. Get Input Values
var airTempF = parseFloat(document.getElementById('airTemp').value);
var concTempF = parseFloat(document.getElementById('concTemp').value);
var humidity = parseFloat(document.getElementById('relHumidity').value);
var windSpeedMph = parseFloat(document.getElementById('windSpeed').value);
// 2. Validation
if (isNaN(airTempF) || isNaN(concTempF) || isNaN(humidity) || isNaN(windSpeedMph)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (humidity 100) {
alert("Relative Humidity must be between 0 and 100.");
return;
}
// 3. Calculation Logic (Menzel Formula via Metric Conversion for Accuracy)
// Formula: E (kg/m^2/h) = 5 * ([Tc + 18]^2.5 – r * [Ta + 18]^2.5) * (V + 4) * 10^-6
// Where Temps are Celsius, V is km/h.
// Convert Inputs to Metric
var airTempC = (airTempF – 32) * (5 / 9);
var concTempC = (concTempF – 32) * (5 / 9);
var windSpeedKmh = windSpeedMph * 1.60934;
var r = humidity / 100; // Decimal humidity
// Calculate Term A: (Tc + 18)^2.5
var termA = Math.pow((concTempC + 18), 2.5);
// Calculate Term B: r * (Ta + 18)^2.5
var termB = r * Math.pow((airTempC + 18), 2.5);
// Calculate Velocity Term: (V + 4)
var termV = windSpeedKmh + 4;
// Calculate Evaporation in Metric (kg/m^2/h)
var evapMetric = 5 * (termA – termB) * termV * 0.000001;
// Convert to Imperial (lb/ft^2/hr)
// 1 kg/m^2 = 0.204816 lb/ft^2
var evapImperial = evapMetric * 0.204816;
// Ensure result isn't negative (physically impossible for evaporation in this context usually, unless condensation)
if (evapImperial 0.2) {
riskClass = "risk-high";
riskText = "HIGH RISK";
recommendation = "Evaporation exceeds 0.2 lb/ft²/hr. Plastic shrinkage cracking is very likely. Implementing immediate evaporation control measures (fogging, wind breaks, cooling concrete) is mandatory.";
} else if (evapImperial >= 0.1) {
riskClass = "risk-moderate";
riskText = "MODERATE RISK";
recommendation = "Evaporation is between 0.1 and 0.2 lb/ft²/hr. Cracking is likely, especially for silica fume or low w/c ratio concrete. Precautionary measures are highly recommended.";
} else {
riskClass = "risk-low";
riskText = "LOW RISK";
recommendation = "Evaporation is below 0.1 lb/ft²/hr. Standard curing procedures are generally sufficient, but monitor conditions if weather changes.";
}
// 5. Update UI
var resultBox = document.getElementById('resultBox');
var resultVal = document.getElementById('evapResult');
var riskIndicator = document.getElementById('riskLevel');
var recText = document.getElementById('recommendationText');
resultBox.style.display = "block";
resultVal.innerHTML = evapImperial.toFixed(3);
riskIndicator.className = "concrete-risk-indicator " + riskClass;
riskIndicator.innerHTML = riskText;
recText.innerHTML = recommendation;
}
Understanding Concrete Evaporation Rates
The evaporation rate of surface moisture from freshly placed concrete is a critical factor in the durability and aesthetic quality of the final slab. If water evaporates from the surface faster than it can be replenished by "bleeding" (the natural upward migration of water), the surface will dry out prematurely.
The Danger: When the evaporation rate exceeds the bleed rate, the concrete surface creates tensile stresses that lead to plastic shrinkage cracking. These are shallow, parallel cracks that appear before the concrete has hardened.
How the Menzel Formula Works
This calculator utilizes the standard equation derived by Menzel (1954), which is the basis for the widely used ACI 305R nomograph. The rate of evaporation depends on four primary environmental factors:
Air Temperature: Warmer air can hold more moisture but also increases the energy available for evaporation.
Concrete Temperature: This is often the most significant driver. Higher concrete temperatures increase the vapor pressure at the surface, accelerating evaporation.
Relative Humidity: Lower humidity increases the gradient between the saturated concrete surface and the air, speeding up moisture loss.
Wind Velocity: Wind removes the layer of saturated air directly above the concrete, allowing dry air to replace it and continuing the evaporation process.
ACI 305R Guidelines and Thresholds
According to the American Concrete Institute (ACI) Committee 305 regarding Hot Weather Concreting, specific thresholds warrant immediate action:
> 0.2 lb/ft²/hr (1.0 kg/m²/h): This is the classic danger zone. Probability of plastic shrinkage cracks is high. Protective measures are mandatory.
> 0.1 lb/ft²/hr (0.5 kg/m²/h): While historically considered safe for conventional concrete, this threshold is critical for modern mixes containing silica fume, fly ash, or fine cements which have lower bleed rates.
How to Prevent Plastic Shrinkage Cracking
If your calculation indicates a Moderate or High risk, consider the following mitigation strategies: