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  • Writer's pictureSi Shen

Time-dependent behaviour of structures (2) – More on creep and shrinkage?

In the previous blog post, I explained the similarities and differences between creep and shrinkage.


It should be noted that creep and shrinkage are considered independent of each other as they are results of different mechanisms. The effects from both are additive. On a cross section basis, creep and shrinkage are normally considered in an average manner ignoring minor differences.


Building on top of the previous blog post, this one explains two questions below:


1. How quickly do the over-time effects take place?


Creep:

The rate at which creep strain takes place slows down as time goes on. About 50% of creep happens in the first 2 months of loading, about 80% in the first year. So creep is heavily front-loaded for a structure designed with long service life.


Shrinkage:

The autogenous shrinkage happens quickly and slows down gradually – it develops 40% of its full scale within a week, 70% in a month, 95% in 1 year and 100% within 3 years.

Drying shrinkage goes on forever as long as the structure is exposed to an environment that can lose moisture (effectively dryer than the structure). Taking 100 years as a benchmark point for 100% scale. It develops about 10% for the first year, 25% at 3 years, 50% at 10 years, 80% at 30 years. Note that it still goes on past the 100 years mark, but since the most onerous structures are designed to approximately 100 years design life, any further shrinkage can be discounted.


2. What measures can designers adopt to reduce shrinkage and creep:

The effects of creep and shrinkage are normally negative. To help reduce the effect of creep and shrinkage, these measures are typically what engineers have control over:

· The impact from creep and shrinkage can be reduced by precast. Precast elements are typically several weeks old or above by the time they are installed and therefore a large proportion of the shrinkage has already taken place, and the strength of concrete when first loaded is a lot higher than in-situ concrete. Precast also takes out almost all the early age thermal effects and part of the shrinkage out of consideration.

· Strength of concrete. Higher grade concrete has higher elastic modulus and hence lower creep strain. Note that higher strength concrete typically has higher cement content, resulting in higher shrinkage as well, but if the stress level is high, the reduction in creep can significantly overweigh increase in shrinkage. Alternatively, instead of higher grade concrete, concrete of the same grade but with higher stiffness aggregates can be used. However, this is not always feasible for practical reasons.

· Cement class. The quicker the concrete hardens, the less complete the chemical reaction is between the cement and water, and the more leftover water is trapped inside the concrete, hence the more voids left in the concrete when the moisture escapes, leading to more creep and drying shrinkage. The type of cement that reacts slower gains strength in a more sustained manner resulting in higher strength in long term. However, one must bear in mind that if the structure is loaded very shortly after concrete is cast, then perhaps rapid hardening cement is more beneficial in helping reducing creep as the stiffness of concrete is higher at the time of loading.

· Putting reinforcement into the structure can significantly reduce creep.


The following other factors have influence over both creep and shrinkage, in terms of both magnitude and speed, but the designer normally has no control over them:

· Humidity of the environment. The more humid it is, the less concrete shrinks. When concrete is exposed to a dry environment, it shrinks because it gradually loses moisture originally trapped inside the concrete. This is called drying shrinkage. When concrete is immersed under water, it actually swells! The same holds true for creep – wetness is helpful in reducing creep. Dry indoors environment makes concrete creep more than wet outdoors environment.

· Area of face exposed to environment. For a given cross sectional area, higher perimeter of exposed face increases level of creep and shrinkage. In other words, the larger area a structural element is exposed to the environment, the quicker moisture escapes and hence the larger creep.

· Temperature of the environment. Higher temperature typically result in quicker drying of concrete thus more shrinkage and creep. The effect of higher temperature on creep is typically insignificant when the concrete is in a ‘normal’ environment. It only starts to become a problem when the concrete is constantly subject to a temperature of 40 degree C or higher.


The following factors have influence over creep only (they are associated with loading only):

· Magnitude of loading on the structure. Creep starts off to be a linear relationship with loading at low levels of loading. The relationship goes exponential when compressive loading exceeds 45% of the concrete strength.

· The timing of loading the structure. The earlier a structure is loaded, the more creep it has.

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