Made to Measure…Cracks

Who are you?:  Souhad Boughanem

What is your role?: I was an EngD Research Engineer

What is your work about?: Getting cement to crack in a controllable way.

I beg your pardon?: Cementitious materials are very brittle.  When subjected to compressive loads, this doesn’t matter, but when in complex/mixed loading conditions or, very rarely, pure tension, a single (wide) crack can cause failure of a component. By tailoring the material, however, it is possible to put in multiple (narrow) cracks, which means that the damaged material can still perform in its engineering application.  This is achieved by the addition of polymeric fibres to the cementitious matrix: the fibres, with a tailored coating hold the material together when it cracks, limiting the width to below 100 µm.

Ok Why? Three key properties are linked to cracking: shrinkage, durability and mechanical performance. If large cracks appear instead of fine cracks once the material is in service, then the material will fail, either mechanically, or through loss of containment (e.g. in the context of a water carrying tunnel). By allowing the material to crack, but in a controlled way, these properties can be managed to produce a high performance material: such materials are called Engineered Cement Composites (ECCs).

And? What I found in my research was that fibre dispersion is very important for achieving ‘multiple-fine cracking’ behaviour.  Fibre clustering within the material means that other areas in the volume are deprived of fibres, which causes the area without fibres to exhibit brittle behaviour: there is no path for stress transfer, except the (comparatively weak) matrix. The stress becomes concentrated, promoting the widening of a single crack until complete failure of the material occurs.  Fibre orientation also plays an important part in the mechanical behaviour of ECC.  A large number of fibres oriented in the tensile loading direction tend to lead to an ECC material with a higher tensile strain in comparison with an ECC material exhibiting randomly oriented fibres. The manufacturing process and rigidity of the fibres can therefore be tailored to control the fibre orientation and dispersion. This can form the basis for optimising ECC performance for a specific application.

So what? When comparing the mechanical behaviour of an ECC material under tensile stress with an ordinary cement material without fibres, an additional tensile strain of 2.4 % is measured, see Figure 1. After the first crack initiates and grows, the ECC material continues to crack, as seen in the successive drop and then increase in stress, until complete failure of the material. This behaviour makes ECCs ‘pseudo-ductile’.

 

cracks

 

Final Thought: Made to measure cracks significantly change the behaviour of cementitious materials, giving rise to exciting possibilities in construction applications.