Cracking. Causes, Reasons of crack control

Cracking. causes, reasons of crack control

György Balázs (Budapest University of Technology and Economics)

Cracking is one of the most important topic for reinforced concrete structures, especially for the historic ones. Cracks occur due to the numerous influencing factors and the complexity of acting loads. They appear in reinforced concrete whenever the tensile strain capacity of concrete has been reached. After the appearance of a crack, relative displacement (slip) develops between concrete and the reinforcement. The two sides of a crack are not parallel and the crack width at the level of concrete surface is larger than the crack width at the level of the reinforcing bar.
Typical crack patterns include vertical cracks at mid-span (that are called flexural cracks) and inclined cracks in the support zones that are called shear cracks.By the analyses of cracks, we distinguish between cracks that appear on the surface and micro cracks that appear in the close vicinity of the reinforcement. The micro cracks were observed experimentally first by a Japanese scholar, Goto, injecting ink in the interfacial zone (Goto, Otsuka, 1971).

The reasons for limiting crack opening are:

1. liquid tightness and gas tightness,
2. limit the risk of corrosion of the reinforcement,
3. Aesthetical reasons, i.e. appearance

The crack control means the observation of causes of the appearance of cracks, the reasons of crack control and the comparison with allowable crack width limits.There are three causes for appearance of cracks in a reinforced concrete structure:

1. the first one is linked to the technological side and includes plastic shrinkage, plastic settlement, non-uniform temperature changes due to the heat of hydration. These phenomena appears in the early ages of the concrete;
2. The Second cause is related to loads, imposed deformations and imposed settlements (in hardened concrete);
3. The third cause is the volumetric changes in hardened concrete due to: temperature differences, drying shrinkage, corrosion of reinforcement, alkali-carbonate reaction (ACR), alkali-silicate reaction (ASR), delayed ettringite formation creep effects and freeze-thaw cycles.

Types of cracks (see Vide and Balazs et al.,)

Observation and monitoring of cracks

Once the cause of the crack has been identified, its width should be assessed and monitored.The measurement of crack widths in laboratory conditions is carried out by LVDTs with sensitivity of 1 micrometer. In practice optical microscopy is often used to measure the crack widths of 100s sensitivity.The most updated solution to measure crack widths is using a microscope embedded into a camera. The actual crack opening is determined by comparing its value to a reference measurement.

Definition of crack width

The definition of crack width is provided by the integral of the differences between the steel and concrete strains taken over the crack spacing. There are more than fifty different approaches for this definition, which differ in the assumptions to consider the strain distribution and bond laws (Balázs, 1993; Borosnyoi-Balazs, 2005):



Cyclic loads and long-term loads induce: an increase of crack widths and a redistribution of stresses within the concrete cross-sections (Balázs, 1991, Balázs et al.,1997, Balázs, 2000).
Steel fibres that can be added to the mix are able to improve the crack pattern in fibre reinforced concrete. By increasing the steel fibre content, the crack width decreases (Kovács, Balázs, 2003¸ Kovács, Balázs, 2004).
Analysis of the state of stresses and deformations under serviceability conditions can be performed by a numerical procedure, as non-linear finite element analyses based on fracture mechanics approaches. Discrete crack models and smeared crack models are used. For smeared crack models the crack is modelled by a crack band.

Deflections Deflection limits must be considered for the following reasons: appearance, integrity and functioning (Balázs, 2013). The general method of calculation deflections is based on the integration of curvatures.