Description of the physical phenomenon of shrinkage

Shrinkage is a spontaneous phenomenon for concrete that occurs throughout the service life of concrete structures. It is associated to the cement paste, since aggregate does not experience a volume contraction, but they influence the phenomenon. Shrinkage is indeed mainly related to evaporation of the absorbed water; thus, to the porosity of the cement paste, which consists of air voids, capillary pores and gel pores. The loss of water results in a contraction of concrete, usually termed as shrinkage. It can indeed been defined as the time-dependent change in volume during an unstressed state at constant environmental temperature [10]. Without restraints, concrete shrinkage will only result in a reduction of the concrete volume, otherwise internal stresses arise, which may lead to cracking. However, the condition of zero internal stresses is almost impossible, since for concrete elements with thickness greater than 3 mm, a moisture gradient takes place [144]; thus leading to the appearance of eigen-stresses. Shrinkage occurs even in the absence of stresses and develops over time, with a higher rate in the period immediately after casting, while it tends to reach an asymptotic value after very long periods of time.

According to the cause, shrinkage may be divided into five different types, namely plastic, thermal, autogenous, carbonatation and drying shrinkage, that will be discussed in the following subsections. A more detailed discussion will be provided for autogenous and drying shrinkage, since the shrinkage strain is usually evaluated as the sum of only these two components, being the most relevant phenomena for the short-term behavior of RC structures under serviceability conditions.

3.2.1.1 Plastic shrinkage

Plastic shrinkage is associated with the rapid water evaporation from the concrete surfaces into the surrounding environment when it is still in the plastic phase. This phenomenon happens in the freshly poured concrete, thus at early-age, being prominent during the setting period. It favors the onset of cracking on the free surface, with consequent drop in the element durability. In more detail, plastic cracks are likely to appear when moisture loss from concrete surfaces is larger than bleed water. Therefore, this danger can be limited by reducing the evaporation rate or increasing the rate of water bleeding. The first is related to relative humidity, air movement velocity, ambient and concrete temperatures;

whereas the latter depends on cement, water and entrained air contents.

3.2.1.2 Thermal shrinkage

Thermal shrinkage is given by the contraction of the material due to the non-adiabatic conditions during the hydration of the cement. The chemical reactions develop heat, with a consequent increase in temperature and expansion of the concrete. Such reactions are progressively slowed down during the setting, and the temperature is lowered up to a value close to that of the external environment, because of heat dissipation through the formwork, so causing the concrete contraction. Thermal shrinkage is usually neglected; it can become important only during setting and for mass concrete structures, where a massive amount of internal heat is generated during the hydration process.

3.2.1.3 Autogenous shrinkage

Autogenous shrinkage, also called chemical shrinkage, is related to the water consumption from the capillary pores due to the chemical reactions of the hydration process of cement. This results in a bulk volume reduction, which happens under isothermal condition and in the absence of moisture exchange with the external ambient. In particular during the hydration process water reacts with the cement particles to create the cement hydration products: capillary pore water, and if it is not enough also the intracrystalline water, are absorbed as the hydration process continues. This process is known as self-desiccation: a negative pressure develops in the pores and it causes the volume reduction.

Autogenous shrinkage can be considered to develop isotropically within the mass of the material, but it is not uniform. Higher autogenous shrinkage develops in the inner parts of the element since chemical reactions are indeed usually stronger in the core of element, because of the development of higher heat of hydration. It can be considered as an intrinsic characteristic of concrete and almost independent of the size of the specimen. The phenomenon evolves in time with similar trend to development of mechanical strength: its increase is very fast in the first days, reaching at 28 days the 60 - 90% of its final value.

Autogenous shrinkage is relatively small in conventional normal strength concrete (about 5%-10% of the maximum drying shrinkage), but can reach high value in case of very low water/cement ratio. The consumption of pore water in

the hydration process is indeed more rapid in case of low water/cement ratio.

Therefore, autogenous shrinkage cannot be neglected for high strength concrete.

Moreover, also the type of the cement and the additives inserted in the admixture influence chemical shrinkage. For example, the presence of alumina cement, high early-strength cement or fine-grained cement tend to increase this phenomenon as well as the addition of blast-furnace slag, silica fume, and expansive agent.

3.2.1.4 Carbonation shrinkage

Carbonation shrinkage is another kind of chemical shrinkage, since it is related to the chemical reaction between calcium hydroxide, produced by the hydration of the cement, and carbon dioxide in the air, to create calcium carbonate. Carbonation shrinkage begins at the surfaces and gradually penetrates into the core; however, carbon dioxide rarely penetrates through concrete surface for more than a few millimeters; therefore carbonation shrinkage can be usually neglected.

3.2.1.5 Drying shrinkage

Drying shrinkage is the most important type of shrinkage and it is often the single rate considered when referring to shrinkage. It is defined as the volumetric reduction due to the evaporation of water to the surrounding environment, with a constant ambient temperature and relative humidity. The process of drying shrinkage is not fully understood, nevertheless it is thought to begin as soon as the absorbed water is lost to the environment. At first, free water evaporates, even if this loss is found to cause insignificant value of shrinkage. Afterwards, the adsorbed water in the capillary pores starts to evaporate, since the internal humidity tends to equilibrate the inferior environmental one. Also intracrystalline water can be involved in the process after water in the capillary pores is all evaporated but concrete is still exposed to drying. A negative capillary pressure develops and compressive forces are induced on the rigid concrete skeleton, resulting in a volume contraction.

Relating with a unique law drying shrinkage to the water loss is very difficult.

It depends on the specimen size and it is far more complicated for real structures, characterized by a size and shape non-uniform throughout.Moreover, it is not isotropic being higher on the external superficies and lower in the core.

Drying shrinkage increases with time and it continues for long period: it usually ends months or years after casting. However, the rate of volume reduction decreases with time. As already said, the member size plays a major role on drying shrinkage: thin members can shrink for months or years, while for the core of a larger member, the drying process may continue throughout its lifetime. In particular the volume-to-surface area ratio of the element influences the development of shrinkage strains; lower volume-to-surface area ratio permits indeed a greater moisture loss.

Drying shrinkage is affected, a part from the shape and the size of the specimens, by all the mechanisms related to the drying process. They can be

grouped into two main groups: the first is related to the environmental conditions, such as relative humidity, ambient temperature and wind velocity, whereas the latter is related to the intrinsic characteristic of the concrete material, such as the content and type of aggregates, the water and cement content as well as the presence of additives.

As far as environmental conditions are concerned, the major role is played by the relative humidity; a low ambient humidity produces larger gradients near the drying surfaces, thus increasing the drying rate.

As regards the intrinsic characteristic of the concrete mix, aggregates influence a lot drying shrinkage. They are inert, due to their low permeability, so they restrict the overall deformations of the material, providing restraining actions to the cement paste that undergoes drying shrinkage. In particular higher aggregate concentration and higher aggregate stiffness result in lower shrinkage strains. Greater water and cement concentrations produce instead higher shrinkage deformations. In case of high water content drying shrinkage increases due to larger amount of evaporable water; whereas higher cement content results in a lager fraction of cement paste in concrete which represents the shrinking part of the material. Considering the water to cement ratio, reducing this value produces a reduced porosity of the cement paste, leading in turn to lower shrinkage strains. Moreover, also the inclusion of additives in the concrete admixture influences the drying shrinkage potential, since they modify the microstructure of the cement paste, as well as the pore structure.

3.2.2 Effects of shrinkage on the behavior of structural elements