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Influence of the transition zone on properties of concrete

The transition zone is a thin layer around large aggregates in a concrete mix. The transition zone of concrete mix will significantly affect concrete properties, concrete elastic behavior. Also, the behavior under tensile loads. Transition zone serves as a bridge between other components of the concrete mix. the transition zone is a bridge between aggregate and bulk matrix(cement paste). The stiffness of the concrete mix is less than the stiffness of aggregate and bulk matrix. The main reason behind this is the bridge or the transition zone. This bridge possesses lower stiffness, and it can crack and fracture before other components of the mix. The main reason for lower stiffness and early fracture is the voids and microcracks in the transition zone. 

Figure 1




Concrete will fail at considerably at lower stresses due to the existence of the transition zone in the concrete mix. Figure no:1 showing that the aggregate and cement paste possess higher ultimate stress compared to the concrete mix. The cracks will extend rapidly at transition zone at stress higher than 40 to 70% of ultimate stress. The rapid spreading of cracks explains the inelastic behavior of concrete at low stresses, whereas the cement paste and aggregate remains elastic until the concrete fracture. 

At a stress larger 70% of the ultimate stress, cracks begin developing in large voids in mortar matrix, cracks will grow gradually with increasing stress. The cracks will spread, and joint cracks originated in the transition zone. As a result, the cracks become continuous, and material rapture. 

Transition zone will affect the concrete durability. Corrosion of steel will adversely affect concrete. Corrosion will reduce the tensile strength of reinforcing steel. Also, the volume of the reinforcing bars will increase, and this will create stresses on concrete and lead to cracks and spalling of concrete. The existence of porous and voids in the transition zone will allow easier ingress of aggressive chemical, which leads to steel corrosion and reducing of concrete durability.  

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