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4.3. Shrinkage Effects
A well know property of concrete is its propensity to shrink as the water content diminishes,
or for the concrete to swell as it absorbs water. This shrinkage and swelling can give rise to
large apparent strain changes that are not related to load or stress changes. The magnitude
of the strains can be several hundred microstrain.
It is difficult to compensate for these unwanted strains. An attempt may be made, or it may
occur naturally, to keep the concrete under a constant condition of water content. But this is
frequently impossible on concrete structures exposed to varying weather conditions.
Sometimes an attempt is made to measure the shrinkage and/or swelling effect by casting a
strain gage inside a concrete block that remains unloaded but exposed to the same
moisture conditions as the active gages. Strains measured on this gage may be used as a
correction.
4.4. Creep Effects
It is also well known that concrete will creep under a sustained load. What may seem to be
a gradually increasing load as evidenced by a gradually increasing strain may, in fact, be
strain due to creeping under a constant sustained load.
On some projects, gages have been cast into concrete blocks in the laboratory and then
kept loaded by means of springs inside a load frame so that the creep phenomenon can be
quantified.
4.5. Effect of Autogenous Growth
In some old concrete, with a particular combination of aggregates and alkaline cements, the
concrete may expand with time as it undergoes a chemical change and recrystallization.
This expansion is rather like creep but in the opposite direction. It is difficult to account for.
