What is concrete creep?

What is concrete creep?

Creep refers to the continuous deformation of a structural member under a constant load. This phenomenon has a significant impact on various materials, particularly concrete. The precise behavior of concrete creep depends on factors such as the characteristics of aggregates, mix design, environmental moisture, cross-sectional area of the members, and the age of the initial load application. However, the overall creep pattern is calculated over time by considering the axial compression of the member.

Definition of Concrete Creep

Concrete creep occurs when the strain in concrete gradually increases over time while the stress caused by the load remains constant. In simpler terms, creep denotes the progressive increase in strain or deformation of a structural component subjected to a constant load.

Depending on the type of concrete, the structural design, and the service load conditions, creep can result in noticeable displacements within the structure. Severe strains caused by creep can result in functional issues, redistribution of stresses, loss of Prestressing, and even failure of concrete structural members. It should be noted that complete reversal of creep is not achievable. Essentially, creep is an irreversible phenomenon that occurs due to the continuous application of the load, even within a single day, resulting in permanent deformation or irreversibility. When concrete is subjected to continuous and constant compression, creep gradually reduces stress over time. This process is commonly known as relaxation. Creep is also referred to as “plastic flow,” “plastic yielding,” or “plastic deformation”.

Creep Mechanism in Concrete

The magnitude of the stress-strain curve depends on several factors. Among these factors, “stress intensity and the duration of applied load” are particularly important. Thus, it is clear that the relationship between stress and strain for concrete is a function of time. Gradual increase in stress without a corresponding increase in strain, occurring over time is known as creep. Based on this principle, creep is defined as “the gradual increase in strain under sustained stress.”

All materials undergo creep under loading conditions, but concrete experiences notable creep under any stress and for a long period. Moreover, concrete creep demonstrates an almost linear behavior, accounting for approximately 30 to 40 percent of its compressive strength. The magnitude of concrete creep is much higher than that of other crystalline materials, except metals, especially during the final stage before failure. Therefore, creep in concrete is considered a distinct rheological phenomenon closely related to the structure of the cement paste.

The Influence of Cement Paste on Concrete

Cement paste plays a significant role in the deformation of concrete. The characteristics of concrete deformation are influenced by the types and proportions of aggregates used. Therefore, it is logical to begin by examining the composition of cement paste and how it affects the way concrete settles and changes, and then explore how the presence of aggregates further influences this behavior.

Cement paste is essentially a mixture of cement particles and water that transforms into a paste through a process called hydration. Creep in concrete can occur due to the flow of the sticky cement paste, the movement of aggregates. However, studies have revealed that a significant portion of water leakage results from the gel formed during cement hydration. The formation of cement paste and the amount of water present are crucial factors that impact the deformation properties of concrete. Cement paste solidifies through both chemical bonding and adsorption force.

The extent of release from colloidal water retention is dependent on the applied pressure and the friction within the capillary pathways. When the applied force is higher, the pressure gradient decreases, resulting in increased moisture diffusion and deformation. The pressure gradient represents the derivative of fluid velocity or the path by which fluid element moves from regions of higher to lower pressure.

 One explanation for the mechanics of creep is based on the theory that colloidal particles rearrange themselves by displacing water within the paste and the capillary pathways. This flow and subsequent displacement of water contribute to the complex behavior observed in concrete creep. Creep occurs only under stress and constant loads. Over time, under continuous stress, water from the cement paste enters the capillary pathways, leading to cement setting.

Characteristics of Creep in Concrete

The characteristics of creep are as follows:

• The final deformation of the member can be three to four times greater than the short-term elastic deformation.
• • The deformation changes nearly in proportion to the intensity of the applied load and has an inverse relationship with the relative strength of the concrete.
• • Upon load removal, only the immediate elastic deformation recovers, and it does not have any effect on the plastic deformation.
• • There is a redistribution of load between concrete and steel, where compressive stresses in steel increase, and steel bears a considerable portion of the load.

The effects of creep are particularly significant in beams, where increased deflection can lead to crack formation and damage to the concrete member. The redistribution of stresses between concrete and steel occurs in the compressed region where cracks have formed, and it has a minor influence on tensile strengthening. Strengthening the compressed region of a flexural member such as a beam or a building roof often aids in preventing deflection caused by creep.

Types of Creep in Concrete

Creep in concrete can be divided into four categories:

• Basic creep: It refers to the gradual increase in strain over time when a constant stress is applied to a concrete sample under conditions of 100% relative humidity.
• Drying Creep: This is the additional creep that occurs when the loaded sample undergoes drying.
• Specific Creep: Specific creep is a term used to describe the creep strain in relation to the applied stress.

Impact of Creep on Concrete

Creep in concrete has the following effects:

• In reinforced concrete beams, creep leads to an increase in deflection over time, which should be considered into account during the design phase.
• In reinforced concrete columns, creep gradually transfers the load from the concrete to the reinforcement.
• Additional load carried by the reinforcement causes the complete development of strength in both the steel and concrete, making creep behavior advantageous in reinforced concrete columns.
• creep in columns subjected to non-axial loads leads to increased deflection and can potentially result in buckling.
• In statically indeterminate structures, creep can reduce the concentration of stresses arising from factors such as shrinkage, temperature variations, or foundation settlement. It restricts internal pressures caused by non-uniform or shrinkage-induced loads in concrete structures.
• In mass concrete, creep can potentially lead to cracking as the concrete undergoes temperature fluctuations caused by hydration heat and subsequent cooling cycles.
• All necessary precautions and essential actions should be taken to prevent temperature rise within large concrete structures like concrete dams and partition walls.
• The increase in temperature within the internal space of a large concrete mass should be controlled by utilizing low-heat cement, reducing the amount of cement, pre-cooling the designed mix components, limiting the height of concrete lifts, and cooling the concrete through water circulation.
• Regarding prestressed concrete, prestressing can potentially reduce creep.

Factors affecting concrete creep

 The extent of creep depends on the following factors:

1-Compressive strength

• The higher the strength of the concrete, the less it tends to deform over time. Moreover, increasing the applied stress on concrete leads to a greater tendency for creep.

2- Aggregates

• Aggregates typically exhibit minimal creep. The majority of creep occurs within the cement paste. However, when the paste experiences creep, it affects the entire structure. Stronger aggregates provide better restraint, resulting in reduced creep.
• The elastic modulus of aggregates also plays a important role in creep. Higher elastic modulus in aggregates leads to lower creep. Lightweight aggregates, due to their lower elastic modulus, demonstrate higher creep compared to conventional aggregates.
• Studies have shown that larger aggregate sizes correspond to lower creep in concrete.

3-Water-to-Cement Ratio

• The water-to-cement ratio influences the size of pores within the cement paste structure. The content of the paste and the quality of the mix design are among the most important factors affecting creep.
• A weaker paste structure tends to result in higher creep.
• Creep escalates with an increase in the water-to-cement ratio.
• All factors that impact the water-to-cement ratio also have an effect on the deformation or creep of concrete.

4-Age of Concrete under Loading

As concrete ages, its tendency to creep decreases. The following table illustrates this relationship:

5- Cement Moisture:

• Increasing the moisture content of cement in concrete enhances creep.

6- Ambient Humidity:

• The humidity of the air affects the moisture level in concrete, either reducing or increasing it. Higher humidity reduces moisture loss and leakage, thus leading to more creep.
• When the relative humidity is low, the drying time of loaded concrete enhances, resulting in increased creep.

7- Cement Type:

• The type of cement has a significant impact on creep, consequently influencing the strength of concrete during the application of loads. In drying conditions, high-furnace slag cement causes greater creep compared to ordinary Portland cement.
• The fineness of cement particles affects both the early-age strength enhancement and creep. Finer cement requires a higher amount of gypsum. In laboratory re-cementation, inadequate cement is produced without adding gypsum, leading to higher levels of creep.

8- Intensity and Duration of Stress

• Many experiments have shown a direct relationship between creep and the applied stress.
• Researchers have discovered that samples which have been cured for 90 days and then loaded for 21 years correspond to constant stress levels of four megapascals, six megapascals, and eight megapascals.

9-Sample Size

• The size of the sample also has an impact on the amount of creep. As the sample size increases, the creep decreases. This is because there is less leakage as the pathway for water discharge becomes longer. resulting in increased internal friction resistance against water flow.

10-Temperature

• The temperature to which the concrete is exposed can have two opposing effects on creep:
• If a concrete member is exposed to temperatures higher than the normal range during the curing process prior to loading, its resistance increases and the creep strain at lower temperatures decreases.
• On the other hand, exposure to high temperatures during the service period can increase creep. The effect of temperature on creep is particularly important in prestressed concrete reactor vessels (PCRVs).

How is concrete creep measured?

The creep behavior is typically determined by measuring the changes in strain over time under a constant load. The device used for conducting creep tests is depicted below.

In this apparatus, a spring assists in maintaining a constant load, ensuring that the specimen remains stable despite undergoing contraction over time. Under such conditions, creep continues for a prolonged duration, though with a decreasing rate as time progresses.

Creep measurement is accompanied by the phenomenon of concrete shrinkage under compressive force. To mitigate the effects of shrinkage and other volumetric changes in concrete, unloaded samples are required. While this test is considered qualitatively valid and produces acceptable results, some researchers argue that shrinkage and creep are interconnected phenomena. They strongly emphasize that these two factors, as assumed in the experiment, lack sufficient credibility. Nevertheless, this test remains the most reliable method for assessing concrete creep.

How is creep calculated?

Various expressions are employed to indicate the final creep value in a concrete element. The relationship between specific creep (creep strain per unit stress) “c” and the time under load “t” is expressed as follows:

The constants “a” and “b” are fixed values. By plotting a graph with “t” on the x-axis and “t/c” on the y-axis, a straight line with a slope of “b” is evident. Using this graph, the constants can be easily computed. When t = a/b and c = b/2, half of the ultimate creep will be achieved at t = a/b.

Final Remarks

The term “creep” in concrete refers to the application of constant loads over time. As time passes and under sustained stress, the stress in the concrete increases. Creep causes a change in shape or deviation of the concrete members. Under continuous loading, the concrete gradually readjusts itself over an extended period, which is known as creep behavior. Shrinkage and creep happen simultaneously. When a sample is relieved from the load, an immediate or gradual reduction in strain occurs, which is called creep recovery. This phenomenon is more commonly observed in structures such as bridges and specific members like beams and slabs that experience constant loading.