Calculator
Example Data Table
| Case | fck (MPa) | RH (%) | h0 (mm) | t0 (days) | t (days) | σ (MPa) | Typical outcome |
|---|---|---|---|---|---|---|---|
| A | 30 | 60 | 200 | 7 | 365 | 8 | Moderate creep coefficient; noticeable long-term strain. |
| B | 40 | 75 | 300 | 14 | 730 | 10 | Lower creep due to higher strength and humidity. |
| C | 25 | 45 | 120 | 3 | 180 | 6 | Higher creep risk; check serviceability carefully. |
Formula Used
The calculator estimates the creep coefficient φ as a product of a basic coefficient and a time-development factor:
- φ = φ0 · β(t, t0)
- φ0 = φRH · φf · φt0
- β(t, t0) = ((t − t0) / ((t − t0) + τ))0.30, where τ = 10 + h0/20
Strains under sustained compressive stress σ use an estimated modulus Ec:
- εe = σ / Ec (instantaneous elastic strain)
- εcr = φ · εe (creep strain)
- εtot = εe + εcr = (1 + φ) · σ / Ec (total sustained strain)
This is a simplified engineering model intended for quick comparisons and preliminary checks. Material testing, reinforcement effects, cracking, and code-specific modifiers can change results.
How to Use This Calculator
- Enter the concrete strength, relative humidity, and member notional size h0.
- Provide the age at loading t0 and the evaluation time t.
- Select a cement class that best matches your mix behavior.
- Input the sustained compressive stress and optional instantaneous deflection.
- Press Calculate to view φ, strains, and the deflection multiplier.
- Use CSV or PDF export to attach results to a design note.
Professional Article
1) Understanding concrete creep in structures
Concrete creep is a time-dependent increase in strain under sustained stress. In buildings, it influences long-term deflection, shortening of columns, and prestress losses. Designers track creep to maintain serviceability, protect finishes, and limit differential movements that can distress cladding and partitions.
2) Typical time scale and why early months matter
Most creep develops during the first months after loading, then slows but continues for years. A one-year check is common for floors, while multi-year checks help for tall buildings. The calculator’s time factor reflects diminishing growth as the member ages.
3) Relative humidity and drying sensitivity
Lower relative humidity increases moisture loss from the paste, raising creep potential. Indoor environments can still be dry due to HVAC operation. For exposed members, seasonal humidity swings may change long-term behavior, so using a representative annual average helps comparisons.
4) Member size, notional thickness, and curing
Thinner sections dry faster, so they generally show higher creep. Notional size h0 is a practical proxy for the drying path length. Better curing delays moisture loss and improves microstructure, often reducing creep-driven movements in slabs and beams.
5) Strength, stiffness, and realistic input values
Higher strength typically means higher modulus and lower elastic strain, which can reduce total deformation. Use verified strength where possible, not only the specified grade. If strengths vary between pours, column shortening estimates should use pour-specific values for better accuracy.
6) Age at loading and construction sequencing
Loading at younger ages increases creep because hydration is still developing. Fast-track schedules can therefore increase long-term sag and shortening. Delaying heavy sustained loads, optimizing shoring and stripping, and maintaining curing practices can materially reduce serviceability risk.
7) Stress level and the service range
Creep models are most dependable at typical service stresses. Very high sustained stresses and significant cracking can change stiffness and cause greater deflection than simple multipliers predict. Use the stress input that matches the sustained portion of loading, not the short-term peak.
8) Using results for documentation and decisions
The creep coefficient φ indicates additional strain relative to initial elastic strain. Total strain supports shortening checks, and the deflection multiplier provides a quick long-term lens for beams and slabs. Export CSV or PDF to capture assumptions, compare scenarios, and support professional design notes.
FAQs
1) What does the creep coefficient φ represent?
φ is the ratio of creep strain to the initial elastic strain under the same sustained stress. For example, φ = 2 means creep strain is about twice the elastic strain.
2) Why does low humidity increase creep?
Lower humidity promotes drying, which alters the paste microstructure and increases time-dependent deformation. Drier exposure usually produces larger long-term strains and deflections.
3) How should I choose the notional size h0?
Use a representative thickness that reflects drying path length for the member. Thinner members generally have smaller h0 and tend to develop higher creep.
4) What is the best age at loading t0 to input?
Use the age when sustained loads begin, such as when shoring is removed or permanent loads are applied. Earlier t0 typically increases predicted creep.
5) Can I use the deflection multiplier for cracked members?
Use it cautiously. Cracking reduces stiffness and can increase long-term deflection beyond a simple (1+φ) multiplier. For critical members, apply a code-based cracked-section method.
6) How accurate is this calculator for design work?
It is intended for preliminary engineering estimates and quick comparisons. Final design should follow the governing standard and use project-specific materials data when available.
7) What inputs most strongly affect the output?
Relative humidity, notional size, age at loading, and strength are major drivers. Sustained stress affects strain linearly through σ/Ec, while φ scales the time-dependent portion.