Shrinkage & Cracking of concrete
Shrinkage Compensation for Concrete
Shrinkage is the volume reduction and the most common cause of cracking in cement-based materials, especially Portland cement. Cracks in concrete are considered an inevitable result during construction. They form during the process of hardening. Here you will find the most common reactions that are responsible for the formation of cracks.
Cracking is an undesired property of cement-based materials. Cracks negatively influence the aesthetics, durability and strength properties of constructions.
Large efforts are undertaken to reduce the number and size of these cracks. It is common practice in the industry to counteract these phenomena with reinforcements, repair mortars, coatings and injections. Those methods are expensive and non-ecological.
It would be far better to reduce or compensate shrinkage and therefore reduce or prevent cracking, ideally making cement-based products crack-free. We will present several additives, that help reducing the formation of cracks altogether.
Shrinkage is the most common cause of cracking in cement-based materials. Statements like “all concrete must crack” or “cracking is one of the most severe problems facing the concrete industry today” illustrate the critical importance of cracking to modern concrete.
One of the main factors causing cracks is the shrinkage of concrete during solidification. This process occurs in all water-containing building materials and increases with their porosity.
If shrinkage induced tensile stresses exceed the tensile strength of the cement-based material, cracks develop. Cement-based materials already have the ability for partial relaxation of these induced tensile stresses, which is called creep relief; thereby the risk for cracking is lowered (see figure below).
Several types of shrinkage are mostly responsible for the formation of cracks, and they are listed below.
Plastic shrinkage
Plastic shrinkage develops on the surface of fresh concrete due to evaporation. This process takes place, while the concrete is still in plastic state. Cracks may be formed up to a depth of 10 cm.
The evaporation of water leads to a lower water/cement ratio at the surface compared to the inner one, leading to stress. Tensions therefore develop and small cracks appear on the surface. These may be the starting points for larger crack developments later on. As it develops in the plastic stage however, it is not noted mostly.
The speed of drying-out of the surface is dependent on temperature, wind velocity and water content.
Damage of this kind can usually be reduced by wetting the surface (e.g. using wet burlaps) or surface coatings with curing compounds.
Chemical shrinkage
In the chemical process of hardening, the volume of the final product is lower than the volume of the starting materials. The volume reduction is about 8 vol. % in general and cannot be reduced in Portland cement.
Water is incorporated in the crystalline structure of the reaction products. As the water evaporates, the concrete is left with major pores that are either filled with excess water if present or lead to the drying out the concrete. This process is called autogenous shrinkage.
Autogenous shrinkage is a self-desiccation process in the concrete and a sub-category of chemical shrinkage. It is difficult to draw the line between chemical and autogenous shrinkage as it depends on the availability of sufficient water. It also depends on the water availability on the surface on the concrete as fresh water can be soaked in under wet conditions.
Autogenous shrinkage depends (also) on the W/C ratio and increases in values under 0,45. In concretes with high cement clinker contents such as HPC and UHPC this will lead to a higher increase in autogenous shrinkage, resulting a in high number of cracks. This may lead to a multiple value of shrinkage cracks.
Most shrinkage of this types occurs in the first weeks and depends of the reactivity of the cement. The faster the reactions, the faster the drying out will be and cracks occur.
Addition of microsilica also enhances this phenomenon. The addition of expanding additives helps reducing it.
Drying shrinkage
Drying shrinkage reflects the drying out of the pores of concrete. This process takes place in a later stage of the concrete drying than the plastic shrinkage.
The loss of water depends on many factors such as the thickness of the construction, the porosity of the concrete, the paste volume and the temperature and relative humidity of the environment. Water evaporation also depends on the porosity of high strength concrete and water contents of the concrete.
In high strength concrete, permeability is very low and so is the drying shrinkage. The fine pore size caused by fine additives such as microsilica, metakaolin and other fine fillers will result in the reduction of pore size and water migration.
Thermal shrinkage
The reaction during concrete hardening liberates heat (heat of hydration) that causes thermal expansion.
In very massive concrete parts (dams and such) the heat generated by chemical reactions cannot be led off in due time. Hence, the core of the pieces has been shown to reach 80°C and above. The heat development depends on the reactivity and amount of the cement used.
Such constructions may take months to cool down. Later cooling results in a shrinkage of the concrete.
One solution to counteract thermal expansion is to cool the concrete parts while in construction. Sometimes water is flown through tubes and even liquid nitrogen is used.
Carbonation shrinkage
Carbonation occurs in hardened concrete due to the reaction of CO2 from the environment on the cement stone.
Water and carbon dioxide form carbon acid that reacts with the free lime developed by the Portland cement, forming calcium carbonate. The reaction depends on the CO2-content in the atmosphere and on humidity to form the acid. Depending on the location these values may vary.
During carbonation the strength of concrete is initially increased due to the filling of the pores with calcium carbonate.
Cracks develop - as mentioned earlier - when shrinkage induced tensile stresses exceed the tensile strength of the cement-based material.
Consequently, there are two possibilities to decrease the cracking tendency; either improvement of the tensile strength characteristics or decrease of the induced tensile stresses. Cement-based materials already have the ability for partial relaxation of these induced tensile stresses, which is called creep relief; thereby the risk for cracking is lowered. Reducing such tensile stresses by generating counteracting compressive stresses through expanding additives is a formulation-based method to reduce the related cracking risk (see figure below).
That means, that a reduction of concrete shrinkage can be achieved through adding expanding agents.
Expansive additives produce a deformation which is opposed to shrinkage. The speed of this volume increase can be balanced in such a way that its timing is about the same as the volume reduction through shrinkage. In this ideal case, the total deformation is compensated, and the cement-based product has a stable volume.
Here are some types of materials known to cause expansion of the concrete:
They expand through different reactions which happen in variable velocity and in durations (see table below).
Type E: Expansion through the formation of the mineral ettringite
Introduction
Ettringite is a Calcium-aluminium-sulphate (CSA) produced by calcium, aluminium and sulphated compounds. When all three components are dissolved in water in the ideal composition, long needles of ettringite are formed.
Mechanism
The reaction with water leads to the formation of the mineral ettringite. This process takes place during the curing in the first 3-4 days and is mostly completed after 28 days. Therefore, no late expansion due to unreacted relicts need to be expected.
Caution: In cement mixtures with a retardation agent, the formation of ettringite may already happen without effect in the liquid stage or influence time and amount of ettringite formation.
The availability of water is crucial for the formation of ettringite. In order to fully react, ettringite needs 32 molecules of water to hydrate, meaning a w/c = 0,85 (water to concrete ration). With the low w/c factors used today, there is often not enough water to build ettringite. The after-treatment must therefore ensure sufficient supply of water.
A disadvantage of Ettringite is its sensitivity to heat. The mineral starts to decompose at 50° C – a temperature that can easily be obtained in massive concrete.
Ettringite-forming expanding agents are mainly used in the United States and Japan. They include:
In all these agents, the reactive components responsible for the formation of ettringite are usually the Calcium-aluminates and sulphates of the cement. Next to water, ettringite needs CaO, Al2O3 and SO3 in solution. These are the same elements as in hydration of cement clinker, GBFS (ground blast furnace slag) and pozzolana.
Type C: Expansion through gas liberation
Introduction
Gas releasing expansion additives consist mostly of aluminium powder producing hydrogen gas which leads to expansion.
Mechanism
Gas-evolving expansion agents react only for a few hours and lose their effectiveness largely as soon as the concrete or mortar sets.
Application
They are therefore used almost exclusively for grouts and injection mortars.
Type M: Expansion based on alkaline earth oxides
Introduction
Calcium oxides and magnesium oxides react with water by forming hydration products that cause an expansion.
Calcium oxide, CaO
The reaction (the transformation of CaO to Ca(OH)2) needs only one water molecule and is therefore well adapted to today’s low w/c values. Calcium oxide can be difficult to handle as its hydration starts spontaneously and the reaction happens partly in the wet stage.
The hydration of CaO is faster than the formation of ettringite. The increase in volume is completed after approximately 24 hours. Sometimes, calcium oxide is combined with systems of the ettringite-type to achieve even greater expansion.
Furthermore, calcium oxide acts as an accelerator to mortars and concrete.
Magnesium oxide, MgO
The process with MgO is slower than the one with CaO and takes about 90 days.
Mechanism
Like CaO, magnesium oxides expand by forming hydration products through the transformation of MgO to Mg(OH)2. This requires only one molecule of water and needs a w/c of only 0,45.
As the solubility of magnesium oxide and magnesium hydroxide is very low, the reaction runs much slower than with CaO. Because of low solubility. it takes place on the surface of the MgO particle and not in solution,
Depending on the production condition of MgO and by modifying the grain size and porosity, one can obtain a very slow or fast reactivity.
Application
MgO causes late expansion if it gets burnt dead, when sintered in OPC clinker in the rotary kiln at around 1450°C. This is the reason for a limit of 5% MgO in EN 195.
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