Calculator Inputs
Example Data Table
| Member | Cover (mm) | Bar Dia (mm) | Spacing (mm) | fs (MPa) | beta | Limit (mm) |
|---|---|---|---|---|---|---|
| Slab | 30 | 16 | 150 | 200 | 1.20 | 0.30 |
| Wall | 40 | 12 | 200 | 180 | 1.30 | 0.25 |
| Beam | 35 | 20 | 120 | 240 | 1.10 | 0.30 |
Formula Used
Flexural crack width estimate (SI): w = 11x10^-6 * beta * fs * (A * a)^(1/3)
- w = estimated crack width (mm)
- beta = factor for strain distribution and bar location
- fs = service steel stress (MPa)
- A = concrete area around one bar (mm^2), approximated as A ~= 2a*s
- a = distance from tension face to bar center (mm)
- s = bar spacing (mm)
Shrinkage crack opening screen: w_sh ~= eps_sh * R * L
- eps_sh = shrinkage strain (microstrain x 1e-6)
- R = restraint factor (0-1)
- L = joint spacing (mm)
How to Use This Calculator
- Select a member type and set an allowable crack width for your exposure.
- Enter cover, bar diameter, and bar spacing for the tension reinforcement.
- Choose a method: input service steel stress directly, or estimate it from service moment.
- Adjust beta if you have project guidance; otherwise keep the default.
- Fill the shrinkage section to screen joint spacing and restraint effects.
- Press Calculate to see results above the form under the header.
- Download CSV or PDF to attach calculations to site inspection records.
Professional Guide to Crack Control
1) Why Crack Control Matters
Crack width influences durability, watertightness, corrosion risk, and finishes. Wide cracks can speed moisture and chloride access to reinforcement and increase maintenance cost. Service-level crack screening helps protect appearance and extend service life for exposed slabs, walls, and beams.
2) Common Crack Types on Construction Sites
Flexural cracks form where tensile demand exceeds concrete capacity under service loads. Shrinkage cracks develop as concrete shortens during drying, especially with restraint. Thermal cracks can follow temperature gradients. This calculator screens flexural and shrinkage mechanisms to support practical site decisions.
3) Service Stress and Reinforcement Layout
Flexural crack width is strongly linked to service steel stress. Lower stress usually means smaller cracks. Reinforcement layout also matters: closer spacing improves crack distribution and limits individual crack opening. For slab checks, a 1000 mm strip is commonly used to keep assumptions consistent. Record the stress basis and load case used for traceable QA notes.
4) Cover, Bar Size, and Spacing Effects
Larger cover increases the distance to bar center and can increase crack width. Smaller bars at closer spacing typically control cracking better than fewer large bars. Typical site ranges are cover 25–50 mm and spacing 100–200 mm, adjusted for exposure, congestion, and finish requirements.
5) Beta Factor and Strain Gradient
The beta factor represents strain distribution effects and bar location within the tension zone. Values near 1.0 suit many general checks, while higher values can reflect steeper strain gradients. If your project provides guidance, use it to maintain consistent acceptance decisions across pours and crews.
6) Shrinkage, Restraint, and Joint Spacing
Drying shrinkage is often 400–800 microstrain, varying with materials and curing. Restraint rises near stiff supports, long pours, or bonded interfaces. Joint spacing is frequently 4–8 m for slabs. Good curing, reduced water content, and timely saw-cuts can lower shrinkage-driven cracking risk and improve panel behavior.
7) Practical Limits for Durability and Appearance
Limits depend on exposure and function. Many general elements use 0.30–0.40 mm for appearance, while aggressive exposure or watertight work may target 0.20–0.25 mm. Always follow your specification and governing standard for acceptance, inspection timing, and repair criteria.
8) Field Workflow and Documentation
Use this tool to compare detailing options: adjust spacing, stress assumptions, curing approach, and joint layout. After placement, combine results with reinforcement inspections and curing records. Update checks if bar spacing changes during coordination. Export CSV or PDF outputs to support QA/QC files and client reporting.
FAQs
1) What allowable crack width should I choose?
Many building elements use 0.30–0.40 mm for appearance, while harsh exposure or watertight work may target 0.20–0.25 mm. Always follow project specifications and governing standards.
2) When should I use the service moment option?
Use it when you do not know service steel stress but you can provide service moment, section dimensions, and steel area. It estimates stress using a simplified cracked-section approach.
3) How do cover and spacing affect crack width?
Greater cover can increase crack width, while tighter bar spacing usually reduces it by improving crack distribution. Adjusting spacing is often the quickest way to improve crack control.
4) What restraint factor should I enter?
Use 0.4–0.8 for typical restrained slabs and walls. Lower values fit isolated panels, and higher values fit long continuous pours or elements locked to stiff supports.
5) What if the governing crack width exceeds the limit?
Reduce service stress, decrease bar spacing, use smaller bars at closer spacing, improve curing, and revise joint spacing or layout. Confirm changes against the project acceptance criteria.
6) Are the results final design approval?
No. This tool provides screening estimates to support site QA/QC and early detailing decisions. Final design checks must follow the governing structural code and project specification.
7) Why are flexural and shrinkage checks both shown?
Flexural cracking reflects service loading and reinforcement stress, while shrinkage cracking reflects movement and restraint. The governing value highlights which mechanism is more critical for your inputs.