What is Creep?
Creep is defined as when strain in a solid increases with time while the stress producing the strain is kept constant.
Creep in concrete structures refers to the time-dependent deformation that occurs under sustained load during its lifespan and use. This phenomenon doesn't include immediate elastic strains caused by loading or changes due to moisture, such as shrinkage or swelling.
Creep is significant in structural engineering because the strain it causes can be several times larger than that from the applied loads, influencing the serviceability of structures. Although creep can continue for decades and tends to decrease over time, a substantial portion of it occurs within the first year.
In prestressed concrete, creep is accounted for during the design phase to mitigate the loss of prestress, ensuring it doesn't lead to structural problems.
What causes Creep in concrete structures?
Creep in concrete structures is influenced by several interconnected factors, primarily stemming from the concrete mix's characteristics, mechanical strength and the timing of load application.
The type and amount of aggregate used play a pivotal role, as aggregates themselves exhibit minimal creep but significantly impact the concrete's overall creep behaviour by restraining the cement paste.
Another crucial factor is the water-cement ratio; higher ratios tend to amplify creep potential by weakening the cement paste. Moreover, the age of the concrete at the time of loading is vital, with younger concrete showing a greater tendency to creep than older, more mature concrete.
To mitigate creep, employing high-grade concrete, incorporating intelligent reinforcement, and carefully timing the application of loads, especially during the early stages of the concrete's life, are effective strategies. It is vital that concrete elements have reached full strength before full loading is applied to reduce early creep, therefore proper curing is essential to construction efficiency.
What are the signs of Creep in concrete structures?
How can I identify Creep in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
How can I prevent Creep in concrete structures?
Preventing creep in concrete structures involves a multifaceted approach, focusing on the mix design and structural strategies.
Optimizing the concrete mix to ensure high mechanical strength by utilizing a low water-cement ratio and incorporating materials like fly ash or slag can reduce the propensity for creep by improving the microstructure and reducing porosity. Using aggregates with a high modulus of elasticity can also help counteract the deformation effects of creep by providing a restraining influence on the cement paste.
Additionally, applying pre-stressing techniques introduces compressive forces that can offset tensile stresses developed from creep, enhancing the overall durability and serviceability of the structure.
Meticulous planning in the design and construction phases, considering the timing and magnitude of load applications, plays a pivotal role in counteracting the effects of creep in concrete structures. This entails a thorough analysis and strategic scheduling of load applications throughout the building's lifecycle to prevent undue stress and deformation. By assessing the structure's expected load-bearing demands and environmental exposure, engineers can determine the optimal timing for applying significant loads, ensuring that the concrete has achieved sufficient strength and maturity to resist creep effectively
This approach might involve phased construction techniques or delaying the addition of non-structural elements until the concrete has undergone initial curing and gained adequate stiffness.
How can I repair the damage from Creep on concrete structures?
Creep must be monitored in order to check that deformation remains acceptable.
Attention must be paid if External post tensioning is applied since this method can induce further creep.