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Concrete Creep: Causes and Prevention

  • June 12, 2024

Although concrete is an extremely durable building material, it isn’t impervious to damage. Careful attention must be given to the construction of a concrete structure to ensure that it can withstand the stress and loads placed upon it. 

One of the potential problems that can affect concrete structures is called concrete creep.

This article will discuss what concrete creep is, its causes and effects, how to prevent it, and more.

Table of Contents

1. What is Concrete Creep?
2. Concrete Creep vs. Concrete Shrinkage
3. Causes of Concrete Creep
4. What Are The Effects of Creep on Concrete Structures?
5. How to Prevent Concrete Creep


What is Concrete Creep?

According to the American Concrete Institute, “Creep is the increased strain or deformation of a structural element under a constant load.”

In layman’s terms, concrete creep can be likened to the sagging of a mattress that conforms to the shape of your body, when it is so drastic that the mattress is visibly sunken. 

There are multiple levels of creep that a concrete structure can undergo

  • Primary creep is the immediate elastic strain or initial creep when the load is first applied to the concrete. 
  • Secondary creep is the slow progress of this creep over time resulting from a sustained load. 
  • Tertiary creep is the accelerated creep that eventually leads to a break or rupture of the structure.
  • Drying creep is when basic creep and shrinkage are exceeded at drying.

Over time, hardened concrete can gradually change shape under constant pressure, causing an elastic deformation.

Concrete creep has been accepted as an inevitable fact for decades. In fact, creep is still calculated in concrete projects to determine whether the structure will sustain the anticipated load. 

The formula for creep strain requires precise mathematical modeling by a trained professional. 

Related: Concrete Testing Types


Concrete Creep vs. Concrete Shrinkage

Creep and shrinkage are two terms used in civil engineering and structural engineering that refer to deformations of concrete over time, but they have some differences. 

Creep is a permanent, time-dependent deformation caused by a constant load. 

Concrete shrinkage is temporary and results from vapor being expelled through the capillary pores within the concrete. Drying shrinkage, plastic shrinkage, and autogenous shrinkage are good examples. 

With creep, the size of the concrete structure does not change, just its shape.

With shrinkage, the concrete volume decreases or increases. 


Causes of Concrete Creep

Several factors affect creep behavior. 

Content of The Concrete

One of the most significant factors affecting creep is what’s in the concrete mix. The content of the concrete, as well as the amount of hydration, can have a direct bearing on the amount of creep that occurs.  

Aggregates

Aggregate has a much lower creep rate than cement paste. This is because it is already in a solid state. 

Of course, not all aggregates are equal. The stronger and heavier aggregates create concrete with a lower creep rate by restraining the paste that creeps under load. 

Water/Cement Ratio

A higher water-to-cement ratio leads to an increased creep rate. Mixing the right proportion of cement paste is critical to reducing creep and increasing the strength of the concrete. 

The water content and chosen cementitious materials of the cement paste will be among the most important factors determining creep.  

Admixtures

Admixtures can help or harm; some increase creep, while others decrease it.

Shrinkage-reducing admixtures can help by changing the microstructure of the cement to one that is more able to sustain the load. 

Age of The Concrete Structure

Concrete strengthens with time. When concrete is introduced to a sustained load early, it will have a higher creep rate. To analyze the potential for creep, engineers use a creep coefficient according to age

Humidity

causes of concrete creep - humidity

Dry concrete under stress has a high creep rate. The creep rate decreases when relative humidity is high in the concrete’s environment.


What Are The Effects of Creep on Concrete Structures?

Creep deformations to the structure can cause cracks and loss of structural integrity. Creep can bend or warp concrete beams and slabs that support the entire building. These deflections from axial load can even cause the bucking of reinforced beams.

Prestressed concrete can lose some of its prestress in accordance with the modulus elasticity of the prestressed steel, which results in a reduction in tensile strength.


How to Prevent Concrete Creep

Several steps can be taken to prevent or at least minimize concrete creep in structures to maintain their strength and longevity. 

(Of course, it can be challenging to be confident of the strength of your concrete. Proper testing should be conducted in order to analyze your concrete mix.)

Perform Creep Testing

Engineers used to have to perform complex and lengthy calculations to analyze the creep potential of their concrete. Several variables affect the outcome of this formula

These variables themselves had formulas to determine their value. Some online calculators are available today. However, simple errors can greatly throw off the results. 

The most reliable way to perform creep testing is using a device that places a concrete sample under stress. This machine will measure a time-dependent rate of creep in relation to the strain it is undergoing. 

Software is also available to provide creep analysis when all variables are known. 

Add Aggregates

Adding coarse aggregates to the cement with low void content and a high modulus of elasticity can lower the creep rate of your concrete structure.

Dense, non-absorptive aggregates like limestone, quartz, granite, basalt, and sandstone work best. 

Lightweight aggregates may be used if the cement carries supplemental binders that increase the strength of concrete as it cures.

Use the Right Type of Concrete

There are many types of concrete, each with unique attributes. Some are better at preventing creep than others. 

Fiber-reinforced concrete has tiny steel fibers within the mix that add much more strength and durability. These fibers could also be made of:

  • Glass
  • Polymers
  • Jute
  • Sisal 

High-performance concrete and high-strength concrete are also good choices since they have some of the highest compressive strength.

Learn more: The 15 Types of Concrete Used in Commercial Construction

Add Reinforcements

Rebar, steel mesh, and fiber reinforcement may not change the creep rate of the concrete itself, but they can mitigate the deformation or cracking that occurs. This can help to increase the longevity and strength of the entire structure. 

Read more: What’s the Purpose of Rebar in Concrete Construction?

Perform Prestressing

Prestressed concrete could be in the form of post-tensioned or pre-tensioned concrete. These methods balance the initial stresses the concrete will undergo and, in effect, reduce the resulting deformation. 

Control The Curing Process

Proper concrete curing cannot be overstressed. Any shortcuts or a failure to adhere to the curing protocol will have a devastating effect on the structure’s creep rate. 

Several steps in the curing process will require great attention to detail to successfully reduce the creep of concrete. 

Protect Concrete From Drying Too Fast in Initial Curing

In this phase, it is imperative that the concrete dries out slowly. There are a couple of ways to accomplish this.

Fogging is one of the most effective but requires a little more labor. 

When you fog your concrete slab, you are essentially changing the humidity content of the air above it. When the air has a high water content, the water in the concrete will evaporate more slowly. 

Another way to keep your concrete from drying too fast is by using evaporation retarders.

These are liquid chemicals that can be sprayed over the surface of the concrete after form strike-off. These chemicals slow down the evaporation rate of the water content in the concrete. 

Keep Concrete Moist in The Final Curing

Maintaining good moisture content during the final curing is also important. 

For instance, “sprinkling” uses a light misting hose to spread water over the concrete slab’s surface. Ponding involves placing actual pools of water over the concrete. 

More often than not, you can use a cover to trap the moisture in the concrete, even if another method has already been applied. Some materials used as a cover include:

  • Burlap
  • Cotton
  • Wet sand
  • Straw

The most popular cover material is plastic sheeting. To make them effective, workers apply white plastic in hot weather and black plastic in cold weather. 

“Concrete Cure” is a sprayable liquid membrane that seals concrete and prevents hydration from escaping. 

Use Non-Absorbent Forms

When building vertical structures like beams, columns, and walls, using non-absorbent forms such as metal or sealed wood is crucial to ensure the form doesn’t steal some moisture from the concrete. 

Keep Concrete Cool

Heat makes water evaporate, so keeping the concrete cool will help maintain its moisture. 

One way to lower the temperature of the concrete is to use cold water and aggregates before mixing them with the cement.  The mix can also be precooled before pouring the concrete into the form. 

If the concrete has embedded piping, cool water circulation throughout this piping can keep the concrete cool during much of the curing process. 

Use High-Pressure Steam Curing

This method of curing structures involves placing them in a closed chamber with high heat and high pressure. It is often used for prefabricated structures such as:

  • Precast panels
  • Columns
  • Cellular blocks

This strategy forces the concrete to cure faster, but in a controlled environment that ensures it is kept moist. 

Allow Structure to Cure Longer

The longer you wait before making a concrete structure sustain a heavy load, the better. Allowing some time to pass will ensure the concrete has high durability. Concrete gets stronger as time goes by and can sustain higher stress levels.


Recap

Concrete creep can be devastating to a structure by causing deformation, cracks, and, when unchecked, total failure. However, these catastrophic events can be avoided by following the guidelines above. 

Experienced and professional commercial contractors are well informed about the tactics to prevent creep and take the time to analyze and test their structures to ensure this.

 
Would you like to make sure that your next project is creep-free? Contact FMP Construction to discuss your options.

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