Creep and shrinkage are much ‘hated’ by designers, especially when put in comparison with steel, which does not have these issues at all. These are two very important long-term properties of concrete, but rarely appears in university textbooks thus widely underappreciated. This blog post compares these two apparently similar properties, with the aim of helping fellow designers gain a much deeper understanding of them both.
Explain creep in a sentence:
When a concrete element is subject to a compression, it has certain elastic strain taken place immediately. If the compression stays there for a long time, the concrete undergoes creep strain, in addition to the elastic one.
Explain shrinkage in a sentence:
Concrete continuously contracts after it has been cast, under a few different mechanisms.
Similarities between the two
Creep and shrinkage happen gradually over a long period of time and therefore are primarily Serviceability Limit State behaviours. The impact from creep should be assessed under quasi-permanent load combination, which is a fancy name for ‘normal (or average) working load’ of a structure. They may bring additional deformation to the structure inducing additional stress, which adds to that from structural loadings.
Creep and shrinkage share some structural effects in common, including the increase of in-span deflection in flexural members and reduction in pre-stress.
Creep and shrinkage have similar root causes. The causes are two parts – water and cement:
1. The escape of moisture trapped in the concrete. Real concrete under microscope is like a sponge, with a lot of cavities that suck in water. This is because typically a lot more water than what is required for chemical reaction is mixed into concrete to make it more flowy so that workers can manoeuvre it and compact it easily. The leftover water after chemical reaction is trapped inside the concrete initially but gradually escape to the environment if it can. This process over time will leave a lot of cavities in the concrete, making it ‘softer’.
2. The tendency to contract over time is a natural property for all cement. All the creep and shrinkage happen in the cement part of concrete and not in the aggregates.
Differences between the two
Creep and shrinkage have some significant differences:
1. Creep and shrinkage are fundamentally different in their mechanisms. Creep is the increase in strain over time induced by a given stress; shrinkage happens regardless of stress. In other words, shrinkage happens regardless of compressive stress in concrete whereas creep can only happen when loaded. The magnitude of creep is corelated to applied stress whereas that of shrinkage is irrelevant to applied stress.
2. Creep and shrinkage typically result in different issues for the structure. Creep increases the shortening of compression members, which results in incremental movement of parts of the structure. This can be significant for brittle cladding or partitions, or induce second order effects into other connected members. From my experience in heavy civil structures, the creep strain is usually 1.5 to 2 times the immediate elastic strain. The typical issue for shrinkage, on the other hand, is that the strain induces tensile stress, or cracks, to the structure when restrained from moving. My previous blog post explains the concept of cracking due to restrained shrinkage (https://www.si-eng.org/blog/crack-control-of-concrete-using-reinforcement-to-achieve-water-tightness-2).
3. In practical design, the way they are accounted for is also different. To take creep into account in practical design, the elastic modulus of concrete is often factored down, i.e. the concrete is effectively softer after creep compared to its initial state. Whereas for shrinkage, a strain is added to the structure, typically not stacked with strains from structural loadings.
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