**3. Intrinsic susceptibility of ribbed bars to corrosion**

It has been recognized earlier that the problem of early distress in reinforced concrete structures started showing up following the start of use of steel reinforcing bars with ribs on the surface.

**Figure 6(d)** shows the start of corrosion at the ribs of TMT bars.

**Figure 6(c)** shows corrosion all over the surface of relatively fresh ribbed TMT bars.

The four bottom bars in **Figure 6(b)** show the start of corrosion preferentially at the ribs of untwisted ribbed bars while the four top bars show corrosion all over the surface of the ribbed bars as a consequence of stressing the bars beyond yield.

These show that:


It cannot be overlooked that the ribs were provided out of a perceived necessity of improved "bond" between rebar and concrete when the rebars were upgraded from low-carbon to medium carbon or high carbon steel for higher strength. The truth is that the presence of ribs on the surface of rebars decreases "bond" between rebars and concrete. But the ribs may provide greater resistance to longitudinal movement of the bars relative to the surrounding concrete. Also, as found in the preceding, the ribs encourage corrosion in rebars; Alekseev [9] and Kar [1, 5].

Whether of the CTD or TMT type, or not, the reasons for ribbed bars of carbon steel being intrinsically susceptible to corrosion at accelerated rates are:


6.the rate of corrosion increases with increasing stress levels; the rate accelerates as the stress or strain approaches yield levels, and the surface becomes unstable once at or beyond yield, whereupon the bars become incapable of being passivated and consequently the process of corrosion becomes unstoppable; Kar [1].

The CTD and TMT processes are in violation of the inherent nature of steel to be ductile and to protect itself; Kar [1].

These CTD and TMT bars of high strength steel have another shortcoming to contend with: "The effect of stresses on corrosion is reflected more distinctly in the mechanical characteristics of the reinforcement, specially of high-strength steels with low ductility." [[10], pp. 203–204].

On the basis of extensive work in Russia, Alekseev [10] commented on the above scenario thus: "the durability of reinforcement specimens with a stepped (deformed) profile may be roughly an order less than that of smooth specimens since the former have stress concentrators on the surface at the bases of projections, which represent sites of preferential formation of cracks." [[10], pp. 221–222].

The preceding explains the reasons behind the intrinsic susceptibility of ribbed bars of steel to corrosion at accelerated rates.

It is the effect of this high susceptibility of ribbed bars to corrosion that led to the observations by Papadakis et al. [19] and Swamy [20], and to the types of early distress in reinforced concrete constructions, as depicted in **Figures 3**–**5**.
