Posted by Eric Boullenois - 22 May, 2019
Cracks in a concrete structure reduce its strength and durability, leading to unplanned repair costs. As explained in Part 1 of this article, cracking is an unavoidable tendency of concrete, in fact there are many reasons why cracks may occur and we explained some of them. Here, we would like to focus on what happens within the concrete structure when a crack occurs, therefore the structure becomes weak and the degradation process is accelerated.
Most cracks are wrinkled for the following reasons:- Load / A,B
- Horizontal movement / C
- Vertical movement / D,E
As illustrated in this diagram, some heavy loads (L) which would usually not harm the integrity of the structure, result in shearing abrasions almost perpendicular to the surface of the concrete. This tendency is all the more pronounced if the crack presents some irregularities. (Fig. a and b)
As a result of deformations and variations of hydraulic and thermal parameters, the lips can move away and come closer; a material entering the crack (grains of sand for example) then causes breaks approximately parallel to the surface of the concrete. (fig c). The lips of the cracks can also move (fig d and e) and lead to wrinkles.
The Diagram illustrates the phenomena caused by the presence of cracks, and their effect in increasing the severity of defects and damage to the structure as soon as they appear.
It is easy clear that cracks have considerable impact on the durability of the structure. However, cracks do not only have mechanical effects. In fact, cracks also allow foreign matter and aggressive substances to penetrate the concrete. These may weaken and damage the concrete and the reinforcements. In this case, it is necessary to intervene as quickly as possible in order to avoid a reduction in the service life of the structure and possible structural risks.
The main substances that can penetrate cracks and the processes that can contribute to the degradation of concrete are:
- Chlorides contained in de-icing salts and marine salts. Through the crack, they can reach reinforcements very quickly and trigger corrosion. This corrosion can be very dangerous as it may occur at a single point on the rebar, causing it to rupture and rendering the reinforcement ineffective.
- The carbonation of concrete, a natural phenomenon, leads to a reduction in the high pH of concrete; which is the reason for the passivation of steel in concrete. Carbonation is accelerated when a crack is opened and can penetrate to the rebars much faster.
- Water is not harmless to concrete. When cracks are present. the humidity of the concrete is significantly increased, also leading to an increase in the corrosion rate of steel. In combination with mechanical effects such as runoff, the water will widen and deepen the crack. In the case of frost, the water in a crack can quickly cause even more severe damage.
In general, a crack makes the concrete and the structure more vulnerable to external effects, accelerates the ageing process and can immediately reduce the mechanical resistance of the structure. Last but not least, cracks reduce the ability of a structure to absorb stress and may lead to collapse. If cracks form, their impact on the strength of the structure should be evaluated and a monitoring plan should be drawn up.
In the third and last part of this article “Why concrete cracks?” we will focus on how to combat cracking and on the most important preventive measures.
