Posted by Eric Boullenois - 04 July, 2019
In view of the Part 1 and Part 2 of this article, should we conclude that the cracking of concrete cannot be avoided and resign ourselves to the premature aging of concrete structures? Certainly not. Action can be taken to prevent appearance of cracks! Some brief information is provided below.
Cracks caused by deformation
Action to prevent deformation is initiated during the design phase and the geological study. Cracking caused by deformation may be prevented by taking the following actions:
- improving the humidity resistance of the binder: use of a curing product or regular watering;
- improving the quality of the aggregates used and more intensive vibration during installation;
- hooping of the concrete by spiral reinforcements;
- increasing the section of parts subject to compression;
- use of prestressing for parts subject to flexion;
- increasing the resistance of concrete to tensile forces;
- good sizing and positioning of reinforcement;
- good adhesion of joints to concrete;
- integration of structural joints in the design.
Cracks caused by hydraulic shrinkage
Cracks caused by hydraulic shrinkage may be limited by:
- increasing the maximal size of aggregates;
- limitation of soluble additives or addition of pozzolan to Portland concrete, which has the effect of reducing the hydraulic shrinkage of the binder.
We often try to reduce hydraulic shrinkage by choosing a suitable cement, improving the compaction, and increasing the size of the aggregates as much as possible. Initial conservation is also important: evaporation must be prevented at least until the end of setting. Cold, dry winds are particularly damaging. They delay setting and increase evaporation .
Cracks caused by thermal shrinkage
Heat shrinking can be prevented by limiting the temperature increase in the concrete to the greatest extent possible. In summer in particular, cold water may be used for mixing and cement and aggregates may be stored in an area protected to the sun. This also contributes to the long-term mechanical strength of concrete.
Cracks caused by swelling
Cracks caused by aggressive substances such as sulphated water, acidic water or pure water, may be prevented by making the concrete impermeable e.g. applying a protective coating.
Specially formulated mortars
After a summary of the causes of cracking in concrete, let us look at the case of specially formulated mortars. As mortars are cement-based, the phenomena are similar to those with concrete. Below we describe some of the measures which may be taken to prevent the cracking of mortars.
- Due to their field of application, the ratio surface/volume of the mortars are often very high and has a strong influence on the hydraulic shrinkage: because of the large exposed surface, the hydraulic surface can reach very high level especially when there is a risk of rapid evaporation. This risk can be avoided by humidifying the support or applying a primer before the mortar, preventing this way the absorption of water by the support and thus reducing hydraulic shrinkage and cracks.
- Before any application of a mortar, check whether it is compatible with the environment where it is to be applied. For example, sulfate resistance must be considered.
- In the case of use of quick setting mortars, the heat of hydration may be important and it is necessary to limit the volume. These mortars tend to be much more sensitive to cracks than normal setting mortars. Developments in recent years have led to the formulation of the MasterEmaco T quick-setting mortar range, which pushes the boundaries of quick setting and hardening products.
- Cracks are often due to the heterogeneity of the mixture binders together with sands and additives, and the presence of points of low resistance where cracking can be initiated. Thanks to the development of a more homogeneous matrix during cement hydration, new mortars less sensitve to cracking are available like e.g. MasterEmaco S.
- Precision grouts for industrial machines must retain the same volume during hardening, to guarantee the perfect positioning of the machine. Products of the MasterFlow range are highly specialized and are formulated to achieve this requirement.
In conclusion, cracking is a phenomenon that is inherent in concretes as well as mortars, although it is more effectively controlled than in the past. With the precautions described above, regular monitoring of the structure and, if necessary, rapid repair of any premature damage, the service life of the structure can be increased and maintenance costs during the life cycle can be minimized.