Microcracking of Concrete
Concrete is considered a two phase materials, namely, the paste phase and the aggregate phase. At a macro level, it is seen that aggregates are dispersed in a matrix of cement paste.
At the microscopic level, the complexities of the concrete begin to show up, particularly in the vicinity of large aggregate particles.
This area can be considered a third phase, the transition zone, which represent the interfacial region between the particles of coarse aggregate and hardened cement paste.
The transition zone is generally a plane of weakness and therefore, has a far greater influence on the mechanical properties of concrete.
Why Micro Cracking Developed in Concrete
Reason for micro crack is listed below-
Internal bleed water tends to accumulate in the vicinity of elongated, flat and large pieces of aggregate. In these locations, the aggregate cement paste transition zone tends to be weak and easily prone to microcrack. This phenomenon is responsible for the shear-bond failure at the surface of the aggregate particles.
Due to drying shrinkage or temperature variation, the transition zone develops micro-cracks even before a structure is loaded. When the structure is loaded and at high stress levels, these micro cracks propagate a bigger crack are formed resulting in failure of the bond.
Because of the presence of microcracks in the transit zone that concrete fails at a considerably lower stress level than the strength of bulk phase or aggregate.
Knowledge of micro-cracking of concrete contributes considerably to the understanding of its properties such as,
- Its inelastic nature
- The descending portion of the stress-strain curve
- The strength under combined, repeated and sustained loading etc.
The stress-strain curve is related to internal cracking.
The non-linearity of the stress-strain relation is due to the propagation of bond and mortar cracks. With the dissimilar material, lack of bond, higher w/c ratio and bleeding water, the transition zone becomes the weakest link in concrete mass.
Generally speaking the micro raking at the transition zone is a general feature of concrete that is fundamentally responsible for reducing the long term durability of concrete.